Your CSS-changing idea is good, and would work very well for large sites that want to allow this level of user-customization but don't want to forego the cache. However, all that does is let you change the colors and fonts and stuff (unless there's something there that I'm not aware of). By changing the actual UBB Style Templates, we can allow the users to change everything from colors to graphics to nav menus to just about anything else that can be customized.
As for the cache thing: At this time, it actually reduces the cache, cuz it doesn't use it at all. Good for your hard-drive space, bad for your processor. However, in order to make a cache-friendly version, someone (maybe LK? i don't remember...) suggested that we actually use different caches for each style. In that case, the cache size would be affected by how many styles you offer - if you offer two choices, the cache size doubles; four choices, it quadruples, and so on.
---Skorpion