subsites with same theme should use static css and image files with same uri.
i said in http://wordpress.org/support/topic/plugin-wp-super-cache-wont-compress-css?replies=10#post-1631276 :
... please improve browser-side caching in case several sites of multisite installation use same theme. they should use same css files and theme images ie with same address. main blog address of them is good, i think, because it is shorter. further improvement is to make url base of these files configurable, it is for admin can use special virtualhost with optimised parameters for that - without logging, with big expire value, should turn off php and deny php files, etc.
and i have(had?) written about this here: http://mu.wordpress.org/forums/topic/16404 "use blogs' themes' styles and images from blog service/main blog's domain".