Refactored theme css, added Modern theme pre version

lib/gitlab/theme.rb

@@ -0,0 +1,15 @@
+module Gitlab
+  class Theme
+    def self.css_class_by_id(id)
+      themes = { 
+        1 => "ui_basic",
+        2 => "ui_mars",
+        3 => "ui_modern"
+      }
+
+      id ||= 1
+
+      return themes[id]
+    end
+  end
+end