I have the same problem, and I missed all the style sheet for all the themes. But when I check it, the style.css file is there in all the themes, and it is in the right position. And my site host tec cannot fix it. is it a Bug? or just some plugins mess up this weak thing!
But the site still runs as good as with the style sheet functional, just I cannot change anything for any themems!
Do I need to re-install the whole WordPress 2.1?
Can anybody help me fix it?