diff --git a/app/controllers/admin/base_controller.rb b/app/controllers/admin/base_controller.rb
index 4b5afbe157..48685db17a 100644
--- a/app/controllers/admin/base_controller.rb
+++ b/app/controllers/admin/base_controller.rb
@@ -7,17 +7,12 @@ module Admin
 
     layout 'admin'
 
-    before_action :set_body_classes
     before_action :set_cache_headers
 
     after_action :verify_authorized
 
     private
 
-    def set_body_classes
-      @body_classes = 'admin'
-    end
-
     def set_cache_headers
       response.cache_control.replace(private: true, no_store: true)
     end
diff --git a/app/controllers/auth/registrations_controller.rb b/app/controllers/auth/registrations_controller.rb
index c12960934e..4d94c80158 100644
--- a/app/controllers/auth/registrations_controller.rb
+++ b/app/controllers/auth/registrations_controller.rb
@@ -11,7 +11,6 @@ class Auth::RegistrationsController < Devise::RegistrationsController
   before_action :configure_sign_up_params, only: [:create]
   before_action :set_sessions, only: [:edit, :update]
   before_action :set_strikes, only: [:edit, :update]
-  before_action :set_body_classes, only: [:new, :create, :edit, :update]
   before_action :require_not_suspended!, only: [:update]
   before_action :set_cache_headers, only: [:edit, :update]
   before_action :set_rules, only: :new
@@ -104,10 +103,6 @@ class Auth::RegistrationsController < Devise::RegistrationsController
 
   private
 
-  def set_body_classes
-    @body_classes = 'admin' if %w(edit update).include?(action_name)
-  end
-
   def set_invite
     @invite = begin
       invite = Invite.find_by(code: invite_code) if invite_code.present?
diff --git a/app/controllers/disputes/base_controller.rb b/app/controllers/disputes/base_controller.rb
index 1054f3db80..dd24a1b740 100644
--- a/app/controllers/disputes/base_controller.rb
+++ b/app/controllers/disputes/base_controller.rb
@@ -7,16 +7,11 @@ class Disputes::BaseController < ApplicationController
 
   skip_before_action :require_functional!
 
-  before_action :set_body_classes
   before_action :authenticate_user!
   before_action :set_cache_headers
 
   private
 
-  def set_body_classes
-    @body_classes = 'admin'
-  end
-
   def set_cache_headers
     response.cache_control.replace(private: true, no_store: true)
   end
diff --git a/app/controllers/filters/statuses_controller.rb b/app/controllers/filters/statuses_controller.rb
index 94993f938b..7ada13f680 100644
--- a/app/controllers/filters/statuses_controller.rb
+++ b/app/controllers/filters/statuses_controller.rb
@@ -6,7 +6,6 @@ class Filters::StatusesController < ApplicationController
   before_action :authenticate_user!
   before_action :set_filter
   before_action :set_status_filters
-  before_action :set_body_classes
   before_action :set_cache_headers
 
   PER_PAGE = 20
@@ -42,10 +41,6 @@ class Filters::StatusesController < ApplicationController
     'remove' if params[:remove]
   end
 
-  def set_body_classes
-    @body_classes = 'admin'
-  end
-
   def set_cache_headers
     response.cache_control.replace(private: true, no_store: true)
   end
diff --git a/app/controllers/filters_controller.rb b/app/controllers/filters_controller.rb
index bd9964426b..8c4e867e93 100644
--- a/app/controllers/filters_controller.rb
+++ b/app/controllers/filters_controller.rb
@@ -5,7 +5,6 @@ class FiltersController < ApplicationController
 
   before_action :authenticate_user!
   before_action :set_filter, only: [:edit, :update, :destroy]
-  before_action :set_body_classes
   before_action :set_cache_headers
 
   def index
@@ -52,10 +51,6 @@ class FiltersController < ApplicationController
     params.require(:custom_filter).permit(:title, :expires_in, :filter_action, context: [], keywords_attributes: [:id, :keyword, :whole_word, :_destroy])
   end
 
-  def set_body_classes
-    @body_classes = 'admin'
-  end
-
   def set_cache_headers
     response.cache_control.replace(private: true, no_store: true)
   end
diff --git a/app/controllers/invites_controller.rb b/app/controllers/invites_controller.rb
index 9bc5164d59..070852695e 100644
--- a/app/controllers/invites_controller.rb
+++ b/app/controllers/invites_controller.rb
@@ -6,7 +6,6 @@ class InvitesController < ApplicationController
   layout 'admin'
 
   before_action :authenticate_user!
-  before_action :set_body_classes
   before_action :set_cache_headers
 
   def index
@@ -47,10 +46,6 @@ class InvitesController < ApplicationController
     params.require(:invite).permit(:max_uses, :expires_in, :autofollow, :comment)
   end
 
-  def set_body_classes
-    @body_classes = 'admin'
-  end
-
   def set_cache_headers
     response.cache_control.replace(private: true, no_store: true)
   end
diff --git a/app/controllers/oauth/authorized_applications_controller.rb b/app/controllers/oauth/authorized_applications_controller.rb
index 7bb22453ca..267409a9ce 100644
--- a/app/controllers/oauth/authorized_applications_controller.rb
+++ b/app/controllers/oauth/authorized_applications_controller.rb
@@ -6,7 +6,6 @@ class Oauth::AuthorizedApplicationsController < Doorkeeper::AuthorizedApplicatio
   before_action :store_current_location
   before_action :authenticate_resource_owner!
   before_action :require_not_suspended!, only: :destroy
-  before_action :set_body_classes
   before_action :set_cache_headers
 
   before_action :set_last_used_at_by_app, only: :index, unless: -> { request.format == :json }
@@ -23,10 +22,6 @@ class Oauth::AuthorizedApplicationsController < Doorkeeper::AuthorizedApplicatio
 
   private
 
-  def set_body_classes
-    @body_classes = 'admin'
-  end
-
   def store_current_location
     store_location_for(:user, request.url)
   end
diff --git a/app/controllers/relationships_controller.rb b/app/controllers/relationships_controller.rb
index dd794f3199..d351afcfb7 100644
--- a/app/controllers/relationships_controller.rb
+++ b/app/controllers/relationships_controller.rb
@@ -6,7 +6,6 @@ class RelationshipsController < ApplicationController
   before_action :authenticate_user!
   before_action :set_accounts, only: :show
   before_action :set_relationships, only: :show
-  before_action :set_body_classes
   before_action :set_cache_headers
 
   helper_method :following_relationship?, :followed_by_relationship?, :mutual_relationship?
@@ -68,10 +67,6 @@ class RelationshipsController < ApplicationController
     end
   end
 
-  def set_body_classes
-    @body_classes = 'admin'
-  end
-
   def set_cache_headers
     response.cache_control.replace(private: true, no_store: true)
   end
diff --git a/app/controllers/settings/base_controller.rb b/app/controllers/settings/base_controller.rb
index f15140aa2b..188334ac23 100644
--- a/app/controllers/settings/base_controller.rb
+++ b/app/controllers/settings/base_controller.rb
@@ -4,15 +4,10 @@ class Settings::BaseController < ApplicationController
   layout 'admin'
 
   before_action :authenticate_user!
-  before_action :set_body_classes
   before_action :set_cache_headers
 
   private
 
-  def set_body_classes
-    @body_classes = 'admin'
-  end
-
   def set_cache_headers
     response.cache_control.replace(private: true, no_store: true)
   end
diff --git a/app/controllers/severed_relationships_controller.rb b/app/controllers/severed_relationships_controller.rb
index 168e85e3fe..965753a26f 100644
--- a/app/controllers/severed_relationships_controller.rb
+++ b/app/controllers/severed_relationships_controller.rb
@@ -4,7 +4,6 @@ class SeveredRelationshipsController < ApplicationController
   layout 'admin'
 
   before_action :authenticate_user!
-  before_action :set_body_classes
   before_action :set_cache_headers
 
   before_action :set_event, only: [:following, :followers]
@@ -51,10 +50,6 @@ class SeveredRelationshipsController < ApplicationController
     account.local? ? account.local_username_and_domain : account.acct
   end
 
-  def set_body_classes
-    @body_classes = 'admin'
-  end
-
   def set_cache_headers
     response.cache_control.replace(private: true, no_store: true)
   end
diff --git a/app/controllers/statuses_cleanup_controller.rb b/app/controllers/statuses_cleanup_controller.rb
index 4a3fc10ca4..e517bf3ae8 100644
--- a/app/controllers/statuses_cleanup_controller.rb
+++ b/app/controllers/statuses_cleanup_controller.rb
@@ -5,7 +5,6 @@ class StatusesCleanupController < ApplicationController
 
   before_action :authenticate_user!
   before_action :set_policy
-  before_action :set_body_classes
   before_action :set_cache_headers
 
   def show; end
@@ -34,10 +33,6 @@ class StatusesCleanupController < ApplicationController
     params.require(:account_statuses_cleanup_policy).permit(:enabled, :min_status_age, :keep_direct, :keep_pinned, :keep_polls, :keep_media, :keep_self_fav, :keep_self_bookmark, :min_favs, :min_reblogs)
   end
 
-  def set_body_classes
-    @body_classes = 'admin'
-  end
-
   def set_cache_headers
     response.cache_control.replace(private: true, no_store: true)
   end
diff --git a/app/helpers/application_helper.rb b/app/helpers/application_helper.rb
index 7c91df8d4f..de00f76d36 100644
--- a/app/helpers/application_helper.rb
+++ b/app/helpers/application_helper.rb
@@ -159,6 +159,7 @@ module ApplicationHelper
 
   def body_classes
     output = body_class_string.split
+    output << content_for(:body_classes)
     output << "theme-#{current_theme.parameterize}"
     output << 'system-font' if current_account&.user&.setting_system_font_ui
     output << (current_account&.user&.setting_reduce_motion ? 'reduce-motion' : 'no-reduce-motion')
diff --git a/app/views/layouts/admin.html.haml b/app/views/layouts/admin.html.haml
index ebd236c628..3f7727cdfb 100644
--- a/app/views/layouts/admin.html.haml
+++ b/app/views/layouts/admin.html.haml
@@ -3,6 +3,8 @@
   = javascript_pack_tag 'public', crossorigin: 'anonymous'
   = javascript_pack_tag 'admin', async: true, crossorigin: 'anonymous'
 
+- content_for :body_classes, 'admin'
+
 - content_for :content do
   .admin-wrapper
     .sidebar-wrapper
diff --git a/spec/helpers/application_helper_spec.rb b/spec/helpers/application_helper_spec.rb
index c2e618c7de..0f78dc82f5 100644
--- a/spec/helpers/application_helper_spec.rb
+++ b/spec/helpers/application_helper_spec.rb
@@ -8,7 +8,16 @@ RSpec.describe ApplicationHelper do
       before { helper.extend controller_helpers }
 
       it 'uses the controller body classes in the result' do
-        expect(helper.body_classes).to match(/modal-layout compose-standalone/)
+        expect(helper.body_classes)
+          .to match(/modal-layout compose-standalone/)
+          .and match(/theme-default/)
+      end
+
+      it 'includes values set via content_for' do
+        helper.content_for(:body_classes) { 'admin' }
+
+        expect(helper.body_classes)
+          .to match(/admin/)
       end
 
       private