users may specify a theme in their preferences

app/helpers/application_helper.rb

@@ -473,7 +473,7 @@ module ApplicationHelper
   # Returns the theme, controller name, and action as css classes for the
   # HTML body.
   def body_css_classes
-    theme = OpenProject::Themes.current_theme
+    theme = OpenProject::Themes.current_theme user: User.current
 
     css = ['theme-' + theme.identifier.to_s]
 

lib/open_project/themes.rb

@@ -45,11 +45,18 @@ module OpenProject
         DefaultTheme.instance
       end
 
-      def current_theme
-        theme(current_theme_identifier)
+      ##
+      # Returns the currently active theme.
+      # Params: options (Hash)
+      #         - user: Users may define a theme in their preferences
+      #                 if the user has done so, return that theme
+      def current_theme(options = {})
+        user_theme = options[:user].try(:preference)
+                                   .try(:[], :theme)
+        theme(user_theme || application_theme_identifier)
       end
 
-      def current_theme_identifier
+      def application_theme_identifier
         Setting.ui_theme.to_s.to_sym.presence
       end
 

lib/open_project/themes/view_helpers.rb

@@ -36,7 +36,7 @@ module OpenProject
       # if none is configured or one cannot be found it returns the default theme
       # which means this helper always returns a OpenProject::Themes::Theme subclass
       def current_theme
-        OpenProject::Themes.current_theme
+        OpenProject::Themes.current_theme user: User.current
       end
 
       # overrides image_tag defined in ActionView::Helpers::AssetTagHelpers (Rails 4)

spec/lib/open_project/themes_spec.rb

@@ -102,30 +102,30 @@ module OpenProject
         theme = Themes.new_theme do |theme|
           theme.identifier = :new_theme
         end
-        Themes.stub(:current_theme_identifier).and_return :new_theme
+        Themes.stub(:application_theme_identifier).and_return :new_theme
         expect(Themes.current_theme).to eq theme
       end
 
       it "returns the default theme if configured theme wasn't found" do
-        Themes.stub(:current_theme_identifier).and_return :missing_theme
+        Themes.stub(:application_theme_identifier).and_return :missing_theme
         expect(Themes.current_theme).to eq Themes.default_theme
       end
     end
 
-    describe '.current_theme_identifier' do
+    describe '.application_theme_identifier' do
       it "normalizes current theme setting to a symbol" do
         Setting.stub(:ui_theme).and_return 'new_theme'
-        expect(Themes.current_theme_identifier).to eq :new_theme
+        expect(Themes.application_theme_identifier).to eq :new_theme
       end
 
       it "returns nil for an empty string" do
         Setting.stub(:ui_theme).and_return ''
-        expect(Themes.current_theme_identifier).to be_nil
+        expect(Themes.application_theme_identifier).to be_nil
       end
 
       it "returns nil for nil" do
         Setting.stub(:ui_theme).and_return nil
-        expect(Themes.current_theme_identifier).to be_nil
+        expect(Themes.application_theme_identifier).to be_nil
       end
     end
   end