What did you do to cause this?
Creating a new Theme.
What page were you on?
/css.php is included in all files.
Describe the defect:
You can't define multiple theme files within HTML::css_link(Request::base() . 'css.php?css=screen&theme=' . settings('theme')); while the css.php offering an explode on $_REQUEST['theme']. The $_REQUEST['theme'] is used as directory name, so the explode has to run on another $_REQUEST variable, e.g. $_REQUEST['theme_files']. Otherwise the theme could be loaded from the config/settings, then you can use multiple theme css files.
Possible solution
Remove the unnessecary explode loop, if only the theme name should be used. Or introduce another QS variable for multiple files.