Okay,
Does the stlyesheets show up fine on the front end?
Also, is the cache/themes/ folder writable? (chmod 777?)
The theme works and the folder ist writeable. I have no problemes in the frontend.
Can you upload the mybb_themes and mybb_stylesheets tables?
This happened to me aswell afer activating the shoutbox plugin (Just a coincidence I think) but I presumed the data had somehow been damaged and reimported the stylesheets, it was fine after that.
all my stylesheets are fine except for the "board message" one and that is added auto by the plugin