I cannot seem to find the thread where people talk about how to fix this 'bug'. I just installed a fresh version of mybb 1.8 on my forum for testing purposes. the default theme doesn't appear to be loading correctly:
I think you are missing the cache files for some reason, check your cache directory (or just search for theme1/ in the source of the page). Do you have a public URL?
I think it was a permissions problems with the cache directory. I had thought I chmod it to 777, but I guess not.