Added basic Theme support.

Theme is set at application level (in Admin -> Settings).
For now, themes can override stylesheets only (application.css).

git-svn-id: http://redmine.rubyforge.org/svn/trunk@818 e93f8b46-1217-0410-a6f0-8f06a7374b81

config/settings.yml

@@ -92,4 +92,6 @@ issue_list_default_columns:
 # multiple values accepted, comma separated
 repositories_encodings:
   default: ''
+ui_theme:
+  default: ''