+ Added ability to provide an absolute path to a CSS file for add_css_file, rather than just a prefix to look for in skin folders. The absolute path will be concatenated last, allowing for plugins to override existing CSS, whatever the skin you're currently using. (Subs-Cache.php)

+ Added two new CSS array variables: $plugins['plugin_name'] and $plugins_dir['plugin_name'] will both redirect to, respectively, plugin_name's URL and plugin_name's path. (Subs-Cache.php)

@ Yes, of course the goal is to (finally) allow plugins to add custom CSS to the main CSS file, avoiding an extra HTTP request or inline CSS when all you want is to add a couple of lines that should be run on all pages. Plus, I never liked that add_plugin_css_file function anyway. ;)
! Typecasting oversight. (Subs-Cache.php)
! Various fixes to the header CSS: notifications should be vertically aligned along with the rest (should only be noticeable in Wine), guest mode in Wilde should no longer break the #upper_section layout is no languages are selectable, and finally, the language select box had some leftover CSS from its pre-select box era. (index.css)