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.
I've added the theme_files query string variable, if unset it will use default, otherwise separate multiple files with a comma like so:
css.php?theme=my_theme&theme_files=normalize,my_theme.
Possible solution
Remove the unnessecary explode loop, if only the theme name should be used. Or introduce another QS variable for multiple files.