diff --git a/app/controllers/auth/sessions_controller.rb b/app/controllers/auth/sessions_controller.rb
index a2fed644fe..c28c0eab8b 100644
--- a/app/controllers/auth/sessions_controller.rb
+++ b/app/controllers/auth/sessions_controller.rb
@@ -78,7 +78,11 @@ class Auth::SessionsController < Devise::SessionsController
   end
 
   def user_params
-    params.require(:user).permit(:email, :password, :otp_attempt, credential: {})
+    params.require(:user).permit(:email, :password, :otp_attempt, :disable_css, credential: {})
+  end
+
+  def login_page_params
+    params.permit(:with_options)
   end
 
   def after_sign_in_path_for(resource)
@@ -113,6 +117,11 @@ class Auth::SessionsController < Devise::SessionsController
     truthy_param?(:continue)
   end
 
+  def with_login_options?
+    login_page_params[:with_options] == '1'
+  end
+  helper_method :with_login_options?
+
   def restart_session
     clear_attempt_from_session
     redirect_to new_user_session_path, alert: I18n.t('devise.failure.timeout')
@@ -151,6 +160,8 @@ class Auth::SessionsController < Devise::SessionsController
     sign_in(user)
     flash.delete(:notice)
 
+    disable_custom_css!(user) if disable_custom_css?
+
     LoginActivity.create(
       user: user,
       success: true,
@@ -162,6 +173,15 @@ class Auth::SessionsController < Devise::SessionsController
     UserMailer.suspicious_sign_in(user, request.remote_ip, request.user_agent, Time.now.utc).deliver_later! if @login_is_suspicious
   end
 
+  def disable_custom_css?
+    user_params[:disable_css].present? && user_params[:disable_css] != '0'
+  end
+
+  def disable_custom_css!(user)
+    user.settings['web.use_custom_css'] = false
+    user.save!
+  end
+
   def suspicious_sign_in?(user)
     SuspiciousSignInDetector.new(user).suspicious?(request)
   end
diff --git a/app/controllers/custom_css_controller.rb b/app/controllers/custom_css_controller.rb
index eb6417698a..f0ff6f8ca7 100644
--- a/app/controllers/custom_css_controller.rb
+++ b/app/controllers/custom_css_controller.rb
@@ -13,7 +13,17 @@ class CustomCssController < ActionController::Base # rubocop:disable Rails/Appli
   def custom_css_styles
     Setting.custom_css
   end
-  helper_method :custom_css_styles
+
+  def user_custom_css?
+    return false if current_user.nil?
+
+    current_user.setting_use_custom_css && current_user.custom_css_text.present?
+  end
+
+  def user_custom_css
+    current_user.custom_css_text
+  end
+  helper_method :custom_css_styles, :user_custom_css?, :user_custom_css
 
   def set_user_roles
     @user_roles = UserRole.providing_styles
diff --git a/app/controllers/settings/preferences/base_controller.rb b/app/controllers/settings/preferences/base_controller.rb
index ce6e2bba44..d3e62fb5d9 100644
--- a/app/controllers/settings/preferences/base_controller.rb
+++ b/app/controllers/settings/preferences/base_controller.rb
@@ -25,7 +25,7 @@ class Settings::Preferences::BaseController < Settings::BaseController
   end
 
   def original_user_params
-    params.require(:user).permit(:locale, :time_zone, chosen_languages: [], settings_attributes: UserSettings.keys)
+    params.require(:user).permit(:locale, :time_zone, :custom_css_text, chosen_languages: [], settings_attributes: UserSettings.keys)
   end
 
   def disabled_visibilities_params
diff --git a/app/controllers/settings/preferences/custom_css_controller.rb b/app/controllers/settings/preferences/custom_css_controller.rb
new file mode 100644
index 0000000000..6a7369ec49
--- /dev/null
+++ b/app/controllers/settings/preferences/custom_css_controller.rb
@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+class Settings::Preferences::CustomCssController < Settings::Preferences::BaseController
+  private
+
+  def after_update_redirect_path
+    settings_preferences_custom_css_path
+  end
+end
diff --git a/app/controllers/user_custom_css_controller.rb b/app/controllers/user_custom_css_controller.rb
new file mode 100644
index 0000000000..2535e07c03
--- /dev/null
+++ b/app/controllers/user_custom_css_controller.rb
@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+class UserCustomCssController < ActionController::Base # rubocop:disable Rails/ApplicationController
+  before_action :authenticate_user!
+
+  def show
+    render content_type: 'text/css'
+  end
+
+  private
+
+  def user_custom_css
+    current_user.custom_css_text
+  end
+  helper_method :user_custom_css
+end
diff --git a/app/helpers/application_helper.rb b/app/helpers/application_helper.rb
index f37a51a864..7779926bae 100644
--- a/app/helpers/application_helper.rb
+++ b/app/helpers/application_helper.rb
@@ -252,6 +252,18 @@ module ApplicationHelper
     full_asset_url(instance_presenter.mascot&.file&.url || frontend_asset_path('images/elephant_ui_plane.svg'))
   end
 
+  def user_custom_css?
+    return false if current_account&.user.nil?
+
+    current_account.user.setting_use_custom_css && current_account.user.custom_css_text.present?
+  end
+
+  def user_custom_css_version
+    return '0' if current_account&.user&.custom_css.nil?
+
+    current_account&.user&.custom_css&.updated_at.to_s
+  end
+
   private
 
   def storage_host_var
diff --git a/app/models/concerns/user/has_settings.rb b/app/models/concerns/user/has_settings.rb
index 63e585ec5e..655e589aae 100644
--- a/app/models/concerns/user/has_settings.rb
+++ b/app/models/concerns/user/has_settings.rb
@@ -283,6 +283,10 @@ module User::HasSettings
     settings['web.hide_favourite_menu']
   end
 
+  def setting_use_custom_css
+    settings['web.use_custom_css']
+  end
+
   def allows_report_emails?
     settings['notification_emails.report']
   end
diff --git a/app/models/custom_css.rb b/app/models/custom_css.rb
new file mode 100644
index 0000000000..470395a96e
--- /dev/null
+++ b/app/models/custom_css.rb
@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+#
+# == Schema Information
+#
+# Table name: custom_csses
+#
+#  id         :bigint(8)        not null, primary key
+#  user_id    :bigint(8)        not null
+#  css        :string           default(""), not null
+#  created_at :datetime         not null
+#  updated_at :datetime         not null
+#
+class CustomCss < ApplicationRecord
+  belongs_to :user
+end
diff --git a/app/models/user.rb b/app/models/user.rb
index 2fd19550ce..eab5f6fd52 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -101,6 +101,8 @@ class User < ApplicationRecord
   accepts_nested_attributes_for :invite_request, reject_if: ->(attributes) { attributes['text'].blank? && !Setting.require_invite_text }
   validates :invite_request, presence: true, on: :create, if: :invite_text_required?
 
+  has_one :custom_css, inverse_of: :user, dependent: :destroy
+
   validates :email, presence: true, email_address: true
 
   validates_with UserEmailValidator, if: -> { ENV['EMAIL_DOMAIN_LISTS_APPLY_AFTER_CONFIRMATION'] == 'true' || !confirmed? }
@@ -227,6 +229,22 @@ class User < ApplicationRecord
     prepare_returning_user!
   end
 
+  def disable_css
+    false
+  end
+
+  def custom_css_text
+    custom_css&.css.to_s
+  end
+
+  def custom_css_text=(val)
+    if custom_css.present?
+      custom_css.update!(css: val)
+    else
+      CustomCss.create!(user: self, css: val)
+    end
+  end
+
   def pending?
     !approved?
   end
diff --git a/app/models/user_settings.rb b/app/models/user_settings.rb
index 2d962cd87c..0a995be81a 100644
--- a/app/models/user_settings.rb
+++ b/app/models/user_settings.rb
@@ -55,6 +55,7 @@ class UserSettings
     setting :use_blurhash, default: true
     setting :use_pending_items, default: false
     setting :use_system_font, default: false
+    setting :use_custom_css, default: false
     setting :content_font_size, default: 'medium', in: %w(medium large x_large xx_large)
     setting :bookmark_category_needed, default: false
     setting :disable_swiping, default: false
diff --git a/app/models/user_settings/setting.rb b/app/models/user_settings/setting.rb
index 0aab0da689..9b1eb2e1ef 100644
--- a/app/models/user_settings/setting.rb
+++ b/app/models/user_settings/setting.rb
@@ -63,6 +63,8 @@ class UserSettings::Setting
     case default_value
     when TrueClass, FalseClass
       ActiveModel::Type::Boolean.new
+    when Integer
+      ActiveModel::Type::Integer.new
     else
       ActiveModel::Type::String.new
     end
diff --git a/app/views/auth/sessions/new.html.haml b/app/views/auth/sessions/new.html.haml
index 89903581a2..fb64bab781 100644
--- a/app/views/auth/sessions/new.html.haml
+++ b/app/views/auth/sessions/new.html.haml
@@ -30,6 +30,15 @@
                 label: t('simple_form.labels.defaults.password'),
                 wrapper: :with_label
 
+    - if with_login_options?
+      .fields-group
+        = f.input :disable_css,
+                  as: :boolean,
+                  hint: false,
+                  input_html: { 'aria-label': t('auth.disable_custom_css') },
+                  label: t('auth.disable_custom_css'),
+                  wrapper: :with_label
+
     .actions
       = f.button :button, t('auth.login'), type: :submit
 
diff --git a/app/views/auth/shared/_links.html.haml b/app/views/auth/shared/_links.html.haml
index 757ef0a090..bd2245e111 100644
--- a/app/views/auth/shared/_links.html.haml
+++ b/app/views/auth/shared/_links.html.haml
@@ -11,6 +11,9 @@
     - if controller_name != 'passwords' && controller_name != 'registrations'
       %li= link_to t('auth.forgot_password'), new_user_password_path
 
+    - if controller_name != 'passwords' && controller_name != 'registrations' && params[:with_options].nil?
+      %li= link_to t('auth.with_login_options'), new_user_session_path(with_options: '1')
+
   - if controller_name != 'confirmations' && (!user_signed_in? || !current_user.confirmed? || current_user.unconfirmed_email.present?)
     %li= link_to t('auth.didnt_get_confirmation'), new_user_confirmation_path
 
diff --git a/app/views/layouts/application.html.haml b/app/views/layouts/application.html.haml
index a6b34c8a36..51a7c3ebf1 100755
--- a/app/views/layouts/application.html.haml
+++ b/app/views/layouts/application.html.haml
@@ -36,6 +36,9 @@
 
     = stylesheet_link_tag custom_css_path, skip_pipeline: true, host: root_url, media: 'all'
 
+    - if user_custom_css?
+      = stylesheet_link_tag user_custom_css_path({ version: user_custom_css_version }), skip_pipeline: true, host: root_url, media: 'all'
+
     = yield :header_tags
 
   %body{ class: body_classes }
diff --git a/app/views/settings/preferences/custom_css/show.html.haml b/app/views/settings/preferences/custom_css/show.html.haml
new file mode 100644
index 0000000000..03ee400093
--- /dev/null
+++ b/app/views/settings/preferences/custom_css/show.html.haml
@@ -0,0 +1,30 @@
+- content_for :page_title do
+  = t('simple_form.labels.form_admin_settings.custom_css')
+
+- content_for :heading_actions do
+  = button_tag t('generic.save_changes'), class: 'button', form: 'edit_preferences'
+
+= simple_form_for current_user, url: settings_preferences_custom_css_path, html: { method: :put, id: 'edit_preferences' } do |f|
+  = render 'shared/error_messages', object: current_user
+
+  = f.simple_fields_for :settings, current_user.settings do |ff|
+    .fields-group
+      = ff.input :'web.use_custom_css',
+                 hint: false,
+                 label: I18n.t('simple_form.labels.defaults.setting_use_custom_css'),
+                 kmyblue: true,
+                 wrapper: :with_label
+
+    .fields-group
+      = f.input :custom_css_text,
+                as: :text,
+                hint: false,
+                input_html: { rows: 12 },
+                label: I18n.t('simple_form.labels.defaults.setting_custom_css'),
+                kmyblue: true,
+                wrapper: :with_label
+
+  %p.hint= t 'simple_form.hints.defaults.setting_custom_css_lead'
+
+  .actions
+    = f.button :button, t('generic.save_changes'), type: :submit
diff --git a/app/views/user_custom_css/show.css.erb b/app/views/user_custom_css/show.css.erb
new file mode 100644
index 0000000000..97ca495248
--- /dev/null
+++ b/app/views/user_custom_css/show.css.erb
@@ -0,0 +1 @@
+<%= raw user_custom_css %>
diff --git a/config/locales/en.yml b/config/locales/en.yml
index 11d55dd1eb..20f45bebc9 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -1418,6 +1418,7 @@ en:
       prefix_sign_up: Sign up on Mastodon today!
       suffix: With an account, you will be able to follow people, post updates and exchange messages with users from any Mastodon server and more!
     didnt_get_confirmation: Didn't receive a confirmation link?
+    disable_custom_css: Disable custom CSS
     dont_have_your_security_key: Don't have your security key?
     forgot_password: Forgot your password?
     invalid_reset_password_token: Password reset token is invalid or expired. Please request a new one.
@@ -1478,6 +1479,7 @@ en:
       view_strikes: View past strikes against your account
     too_fast: Form submitted too fast, try again.
     use_security_key: Use security key
+    with_login_options: Will you disable your custom css?
   bookmark_categories:
     errors:
       limit: Bookmark category limit
@@ -1906,6 +1908,7 @@ en:
       too_few_options: must have more than one item
       too_many_options: can't contain more than %{max} items
   preferences:
+    custom_css: Custom css
     does_not_search: The full-text search feature is not available on this server. Instead, your posts will be searched according to this setting on other kmyblue servers.
     dtl: Deep timeline
     dtl_hint: 'You can join deep timeline with #%{tag} tag. Following settings make convenient to use deep timeline.'
diff --git a/config/locales/ja.yml b/config/locales/ja.yml
index ebe7c3fe73..37944a42ac 100644
--- a/config/locales/ja.yml
+++ b/config/locales/ja.yml
@@ -1329,6 +1329,7 @@ ja:
       prefix_sign_up: 今すぐMastodonを始めよう！
       suffix: アカウントがあれば、どんなMastodon互換サーバーのユーザーでもフォローしたりメッセージをやり取りできるようになります！
     didnt_get_confirmation: 確認メールを受信できない場合は
+    disable_custom_css: カスタムCSSを無効化する
     dont_have_your_security_key: セキュリティキーを持っていませんか？
     forgot_password: パスワードをお忘れですか？
     invalid_reset_password_token: パスワードリセットトークンが正しくないか期限切れです。もう一度リクエストしてください。
@@ -1384,6 +1385,7 @@ ja:
       view_strikes: 過去のストライクを表示
     too_fast: フォームの送信が速すぎます。もう一度やり直してください。
     use_security_key: セキュリティキーを使用
+    with_login_options: カスタムCSSを無効化しますか？
   challenge:
     confirm: 続ける
     hint_html: 以後1時間はパスワードの再入力を求めません
@@ -1782,6 +1784,7 @@ ja:
       too_few_options: は複数必要です
       too_many_options: は%{max}個までです
   preferences:
+    custom_css: カスタムCSS
     does_not_search: このサーバーでは全文検索機能を利用できません。代わりに、他のkmyblueサーバーであなたの投稿がこの設定に従って検索されます。
     dtl: ディープタイムライン
     dtl_hint: "#%{tag} ハッシュタグに参加することで、ディープタイムラインに投稿できます。ここではディープタイムラインを利用しやすくするための設定ができます。"
diff --git a/config/locales/simple_form.en.yml b/config/locales/simple_form.en.yml
index 51f7c0ac8a..8a89b61a5e 100644
--- a/config/locales/simple_form.en.yml
+++ b/config/locales/simple_form.en.yml
@@ -62,6 +62,7 @@ en:
         setting_allow_quote: Subdued quotes are allowed regardless of this setting; you can quote freely from any source except kmyblue!
         setting_always_send_emails: Normally e-mail notifications won't be sent when you are actively using Mastodon
         setting_bookmark_category_needed: When removing from all category, unbookmarked automatically
+        setting_custom_css_lead: 'Be sure to remember: In the unlikely event that you make a mistake in entering your custom CSS and the screen does not display properly, you can disable your custom CSS from the link at the bottom of the sign-in screen. Open the sign-in screen in private mode of your browser, for example, and disable it.'
         setting_default_searchability: On kmyblue and Fedibird, the search is based on the search permission setting; on Misskey, all public, local public, and non-public posts are searched regardless of this setting; on Mastodon and Firefish, instead of search permission, the "Make public posts freely searchable on other servers" setting in the profile settings is applied. In Mastodon and Firefish, the "Make public posts freely searchable on other servers" setting in the profile settings is applied instead of the search permission.
         setting_default_sensitive: Sensitive media is hidden by default and can be revealed with a click
         setting_disallow_unlisted_public_searchability: この設定を有効にすると、非収載投稿と検索範囲「誰でも」は両立できず不特定多数からの検索が不可になります。Fedibirdと同じ挙動になります
@@ -263,6 +264,7 @@ en:
           medium: Default
           x_large: Large large
           xx_large: Large large large
+        setting_custom_css: Custom CSS
         setting_default_language: Posting language
         setting_default_privacy: Posting privacy
         setting_default_reblog_privacy: Reblogging privacy
@@ -325,6 +327,7 @@ en:
         setting_trends: Show today's trends
         setting_unfollow_modal: Show confirmation dialog before unfollowing someone
         setting_use_blurhash: Show colorful gradients for hidden media
+        setting_use_custom_css: Enable custom CSS
         setting_use_pending_items: Slow mode
         setting_use_public_index: Include permitted accounts post to results of search
         severity: Severity
diff --git a/config/locales/simple_form.ja.yml b/config/locales/simple_form.ja.yml
index d5156b764e..16b44eba98 100644
--- a/config/locales/simple_form.ja.yml
+++ b/config/locales/simple_form.ja.yml
@@ -62,6 +62,7 @@ ja:
         setting_allow_quote: ひかえめな引用はこの設定に関わらず可能です。kmyblue以外からは自由に引用できます
         setting_always_send_emails: 通常、Mastodon からメール通知は行われません。
         setting_bookmark_category_needed: すべてのカテゴリから削除したとき、ブックマークが自動で外れるようになります
+        setting_custom_css_lead: '必ず覚えてください: 万が一カスタムCSSの入力を誤り、画面が正常に表示されなくなった場合は、サインイン画面の下にあるリンクよりカスタムCSSを無効化することができます。ブラウザのプライベートモードなどでサインイン画面を開き、無効化してください。'
         setting_default_searchability: kmyblue・Fedibirdでは検索許可設定に基づき検索されます。Misskeyでは当設定に関係なく、全ての公開・ローカル公開・非収載投稿が検索されます。Mastodon・Firefishでは検索許可の代わりにプロフィール設定の「公開投稿を他のサーバーで自由に検索できるようにする」設定が適用されます
         setting_default_sensitive: 閲覧注意状態のメディアはデフォルトでは内容が伏せられ、クリックして初めて閲覧できるようになります
         setting_disallow_unlisted_public_searchability: この設定を有効にすると、非収載投稿と検索範囲「誰でも」は両立できず不特定多数からの検索が不可になります。Fedibirdと同じ挙動になります
@@ -263,6 +264,7 @@ ja:
           medium: デフォルト
           x_large: 大きい大きい
           xx_large: 大きい大きい大きい
+        setting_custom_css: カスタムCSS
         setting_default_language: 投稿する言語
         setting_default_privacy: 投稿の公開範囲
         setting_default_reblog_privacy: BTの公開範囲
@@ -325,6 +327,7 @@ ja:
         setting_trends: 本日のトレンドタグを表示する
         setting_unfollow_modal: フォローを解除する前に確認ダイアログを表示する
         setting_use_blurhash: 非表示のメディアを色付きのぼかしで表示する
+        setting_use_custom_css: カスタムCSSを有効にする
         setting_use_pending_items: 手動更新モード
         setting_use_public_index: Mastodonの標準設定によって検索が許可されたアカウントの公開投稿を検索結果に含める
         severity: 重大性
diff --git a/config/navigation.rb b/config/navigation.rb
index 9ace656bbc..87d6d7dbec 100644
--- a/config/navigation.rb
+++ b/config/navigation.rb
@@ -18,6 +18,7 @@ SimpleNavigation::Configuration.run do |navigation|
       s.item :appearance, safe_join([material_symbol('computer'), t('settings.appearance')]), settings_preferences_appearance_path
       s.item :notifications, safe_join([material_symbol('mail'), t('settings.notifications')]), settings_preferences_notifications_path
       s.item :reaching, safe_join([material_symbol('search'), t('preferences.reaching')]), settings_preferences_reaching_path
+      s.item :custom_css, safe_join([material_symbol('inbox'), t('preferences.custom_css')]), settings_preferences_custom_css_path
       s.item :other, safe_join([material_symbol('settings'), t('preferences.other')]), settings_preferences_other_path
     end
 
diff --git a/config/routes.rb b/config/routes.rb
index 800118caba..00842a6cce 100644
--- a/config/routes.rb
+++ b/config/routes.rb
@@ -87,6 +87,7 @@ Rails.application.routes.draw do
   get 'manifest', to: 'manifests#show', defaults: { format: 'json' }
   get 'intent', to: 'intents#show'
   get 'custom.css', to: 'custom_css#show', as: :custom_css
+  get 'user_custom.css', to: 'user_custom_css#show', as: :user_custom_css
 
   get 'remote_interaction_helper', to: 'remote_interaction_helper#index'
 
diff --git a/config/routes/settings.rb b/config/routes/settings.rb
index 6018415fc1..ff5ac33bc7 100644
--- a/config/routes/settings.rb
+++ b/config/routes/settings.rb
@@ -11,6 +11,7 @@ namespace :settings do
     resource :appearance, only: [:show, :update], controller: :appearance
     resource :notifications, only: [:show, :update]
     resource :reaching, only: [:show, :update], controller: :reaching
+    resource :custom_css, only: [:show, :update], controller: :custom_css
     resource :other, only: [:show, :update], controller: :other
   end
 
diff --git a/db/migrate/20240828123604_create_custom_csses.rb b/db/migrate/20240828123604_create_custom_csses.rb
new file mode 100644
index 0000000000..5455eb2b61
--- /dev/null
+++ b/db/migrate/20240828123604_create_custom_csses.rb
@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+class CreateCustomCsses < ActiveRecord::Migration[7.1]
+  def change
+    create_table :custom_csses do |t|
+      t.belongs_to :user, foreign_key: { on_delete: :cascade }, null: false
+      t.string :css, null: false, default: ''
+
+      t.timestamps
+    end
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index 0ddb1d4709..5e21d3b1dd 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.1].define(version: 2024_08_08_125420) do
+ActiveRecord::Schema[7.1].define(version: 2024_08_28_123604) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
 
@@ -479,6 +479,14 @@ ActiveRecord::Schema[7.1].define(version: 2024_08_08_125420) do
     t.index ["uri"], name: "index_conversations_on_uri", unique: true, opclass: :text_pattern_ops, where: "(uri IS NOT NULL)"
   end
 
+  create_table "custom_csses", force: :cascade do |t|
+    t.bigint "user_id", null: false
+    t.string "css", default: "", null: false
+    t.datetime "created_at", null: false
+    t.datetime "updated_at", null: false
+    t.index ["user_id"], name: "index_custom_csses_on_user_id"
+  end
+
   create_table "custom_emoji_categories", force: :cascade do |t|
     t.string "name"
     t.datetime "created_at", precision: nil, null: false
@@ -1678,6 +1686,7 @@ ActiveRecord::Schema[7.1].define(version: 2024_08_08_125420) do
   add_foreign_key "circles", "accounts", on_delete: :cascade
   add_foreign_key "conversation_mutes", "accounts", name: "fk_225b4212bb", on_delete: :cascade
   add_foreign_key "conversation_mutes", "conversations", on_delete: :cascade
+  add_foreign_key "custom_csses", "users", on_delete: :cascade
   add_foreign_key "custom_filter_keywords", "custom_filters", on_delete: :cascade
   add_foreign_key "custom_filter_statuses", "custom_filters", on_delete: :cascade
   add_foreign_key "custom_filter_statuses", "statuses", on_delete: :cascade
diff --git a/lib/tasks/dangerous.rake b/lib/tasks/dangerous.rake
index c2ca637645..fcc2a7b52c 100644
--- a/lib/tasks/dangerous.rake
+++ b/lib/tasks/dangerous.rake
@@ -14,6 +14,7 @@ namespace :dangerous do
     end
 
     target_migrations = %w(
+      20240828123604
       20240709063700
       20240426233435
       20240426000034
@@ -113,6 +114,7 @@ namespace :dangerous do
       circles
       circle_accounts
       circle_statuses
+      custom_csses
       emoji_reactions
       friend_domains
       instance_infos
diff --git a/spec/controllers/auth/sessions_controller_spec.rb b/spec/controllers/auth/sessions_controller_spec.rb
index 9a94e5e1a1..5b39f97ae0 100644
--- a/spec/controllers/auth/sessions_controller_spec.rb
+++ b/spec/controllers/auth/sessions_controller_spec.rb
@@ -203,6 +203,45 @@ RSpec.describe Auth::SessionsController do
           end
         end
       end
+
+      context 'with custom css' do
+        let(:params) { {} }
+
+        before do
+          user.settings['web.use_custom_css'] = true
+          user.save!
+
+          post :create, params: { user: { email: user.email, password: user.password }.merge(params) }
+        end
+
+        context 'when does not reset custom css' do
+          let(:params) { { disable_css: '0' } }
+
+          it 'custom css is enabled' do
+            expect(response).to redirect_to(root_path)
+            expect(controller.current_user).to eq user
+            expect(user.reload.setting_use_custom_css).to be true
+          end
+        end
+
+        context 'when reset custom css' do
+          let(:params) { { disable_css: '1' } }
+
+          it 'custom css is disabled' do
+            expect(response).to redirect_to(root_path)
+            expect(controller.current_user).to eq user
+            expect(user.reload.setting_use_custom_css).to be false
+          end
+        end
+
+        context 'when does not specify about custom css' do
+          it 'custom css is enabled' do
+            expect(response).to redirect_to(root_path)
+            expect(controller.current_user).to eq user
+            expect(user.reload.setting_use_custom_css).to be true
+          end
+        end
+      end
     end
 
     context 'when using two-factor authentication' do
diff --git a/spec/requests/user_custom_css_spec.rb b/spec/requests/user_custom_css_spec.rb
new file mode 100644
index 0000000000..a36afc0e6e
--- /dev/null
+++ b/spec/requests/user_custom_css_spec.rb
@@ -0,0 +1,48 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe 'User custom CSS' do
+  let(:user)       { Fabricate(:user) }
+  let(:custom_css) { '* { display: none !important; }' }
+
+  describe 'GET /user_custom.css' do
+    context 'without sign in' do
+      it 'returns 422' do
+        get '/user_custom.css'
+
+        expect(response).to have_http_status(401)
+      end
+    end
+
+    context 'with sign in but custom css is not enabled' do
+      before do
+        user.update!(custom_css_text: custom_css)
+        sign_in user
+      end
+
+      it 'returns custom css' do
+        get '/user_custom.css'
+
+        expect(response).to have_http_status(200)
+        expect(response.content_type).to include 'text/css'
+        expect(response.body.strip).to eq custom_css
+      end
+    end
+
+    context 'with sign in and custom css is enabled' do
+      before do
+        user.update!(custom_css_text: custom_css, settings: { 'web.use_custom_css': true })
+        sign_in user
+      end
+
+      it 'returns custom css' do
+        get '/user_custom.css'
+
+        expect(response).to have_http_status(200)
+        expect(response.content_type).to include 'text/css'
+        expect(response.body.strip).to eq custom_css
+      end
+    end
+  end
+end