Add Dark Mode support to the image_tag helper

Changelog: added

app/helpers/lazy_image_tag_helper.rb

 # frozen_string_literal: true
 
 module LazyImageTagHelper
+  include PreferencesHelper
+
   def placeholder_image
     "data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw=="
   end
 
   # Override the default ActionView `image_tag` helper to support lazy-loading
   def image_tag(source, options = {})
+    source = options[:dark_variant] if options[:dark_variant] && user_application_dark_mode?
     options = options.symbolize_keys
 
     unless options.delete(:lazy) == false

app/helpers/preferences_helper.rb

@@ -62,6 +62,10 @@ module PreferencesHelper
     @user_application_theme ||= Gitlab::Themes.for_user(current_user).css_class
   end
 
+  def user_application_dark_mode?
+    user_application_theme == 'gl-dark'
+  end
+
   def user_application_theme_css_filename
     @user_application_theme_css_filename ||= Gitlab::Themes.for_user(current_user).css_filename
   end

spec/helpers/preferences_helper_spec.rb

@@ -96,6 +96,30 @@ RSpec.describe PreferencesHelper do
     end
   end
 
+  describe '#user_application_dark_mode?' do
+    context 'with a user' do
+      it "returns true if user's selected dark theme" do
+        stub_user(theme_id: 11)
+
+        expect(helper.user_application_dark_mode?).to eq true
+      end
+
+      it "returns false if user's selected any light theme" do
+        stub_user(theme_id: 1)
+
+        expect(helper.user_application_dark_mode?).to eq false
+      end
+    end
+
+    context 'without a user' do
+      it 'returns false' do
+        stub_user
+
+        expect(helper.user_application_dark_mode?).to eq false
+      end
+    end
+  end
+
   describe '#user_color_scheme' do
     context 'with a user' do
       it "returns user's scheme's css_class" do