You probably could create directories in your images directory with the same name as the style sheets that each contained a complete set of graphics to be used with that style sheet. Then when the send_header function is called in your ubbt.inc.php script you could set the $config['images'] variable to include the name of the style sheet's directory... $config['images'] = "{$config['images']/$StyleSheet"; so that from there out all calls to images went to that style sheet's directory.
I did something like this somewhere... actually think it was a few site revamps ago at my main site.
Edit note: Now that templates are used it would be kinda neat to use different templates based on the style sheet too... wooo just think of the number of combinations... billions and billions and billions... wait that's Carl Sagan... ok at least a half dozen or more... lol
