diff --git a/app/helpers/application_helper.rb b/app/helpers/application_helper.rb
index 327901e4e2..9eb0429387 100644
--- a/app/helpers/application_helper.rb
+++ b/app/helpers/application_helper.rb
@@ -27,11 +27,6 @@ module ApplicationHelper
     Setting.open_deletion
   end
 
-  def add_rtl_body_class(other_classes)
-    other_classes = "#{other_classes} rtl" if locale_direction == 'rtl'
-    other_classes
-  end
-
   def locale_direction
     if [:ar, :fa, :he].include?(I18n.locale)
       'rtl'
@@ -77,4 +72,13 @@ module ApplicationHelper
   def react_component(name, props = {})
     content_tag(:div, nil, data: { component: name.to_s.camelcase, props: Oj.dump(props) })
   end
+
+  def body_classes
+    output = (@body_classes || '').split(' ')
+    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'
+    output << 'rtl' if locale_direction == 'rtl'
+    output.reject(&:blank?).join(' ')
+  end
 end
diff --git a/app/views/layouts/application.html.haml b/app/views/layouts/application.html.haml
index 68a903197b..4368642376 100755
--- a/app/views/layouts/application.html.haml
+++ b/app/views/layouts/application.html.haml
@@ -24,9 +24,5 @@
 
     = yield :header_tags
 
-  - body_classes ||= @body_classes || ''
-  - body_classes += ' system-font' if current_account&.user&.setting_system_font_ui
-  - body_classes += current_account&.user&.setting_reduce_motion ? ' reduce-motion' : ' no-reduce-motion'
-
-  %body{ class: add_rtl_body_class(body_classes) }
+  %body{ class: body_classes }
     = content_for?(:content) ? yield(:content) : yield
diff --git a/config/locales/en.yml b/config/locales/en.yml
index 5e127dc713..d8a1086e47 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -350,6 +350,9 @@ en:
       contact_information:
         email: Business e-mail
         username: Contact username
+      custom_css:
+        desc_html: Modify the look with CSS loaded on every page
+        title: Custom CSS
       hero:
         desc_html: Displayed on the frontpage. At least 600x100px recommended. When not set, falls back to instance thumbnail
         title: Hero image
diff --git a/spec/helpers/application_helper_spec.rb b/spec/helpers/application_helper_spec.rb
index ac54f1f703..3ccd96f446 100644
--- a/spec/helpers/application_helper_spec.rb
+++ b/spec/helpers/application_helper_spec.rb
@@ -17,7 +17,7 @@ describe ApplicationHelper do
     end
   end
 
-  describe 'add_rtl_body_class' do
+  describe 'locale_direction' do
     around do |example|
       current_locale = I18n.locale
       example.run
@@ -26,22 +26,22 @@ describe ApplicationHelper do
 
     it 'adds rtl body class if locale is Arabic' do
       I18n.locale = :ar
-      expect(helper.add_rtl_body_class('other classes')).to eq 'other classes rtl'
+      expect(helper.locale_direction).to eq 'rtl'
     end
 
     it 'adds rtl body class if locale is Farsi' do
       I18n.locale = :fa
-      expect(helper.add_rtl_body_class('other classes')).to eq 'other classes rtl'
+      expect(helper.locale_direction).to eq 'rtl'
     end
 
     it 'adds rtl if locale is Hebrew' do
       I18n.locale = :he
-      expect(helper.add_rtl_body_class('other classes')).to eq 'other classes rtl'
+      expect(helper.locale_direction).to eq 'rtl'
     end
 
     it 'does not add rtl if locale is Thai' do
       I18n.locale = :th
-      expect(helper.add_rtl_body_class('other classes')).to eq 'other classes'
+      expect(helper.locale_direction).to_not eq 'rtl'
     end
   end