I'm not entirely sure what you're asking... I believe there was a Vista-ish theme for the N8x0, so you might be able to modify some of the art from that if you're looking to create a Win7 one. I'm not sure what you mean by a "Linux theme", as that's kind of a contradiction in terms (tell me what "Linux" looks like... :P) - not to mention the N900 runs Linux, and therefore is by definition using a "Linux theme".
If, on the other hand, you mean a desktop theme using N900 graphics, then I'm sure there's one out there, but Google is Your Friend too.
Cheers