.com - as if you type a keyword into the toolbar of most browsers by default they jump to the .com address and not the .co.uk (Or any other regional suffix) so in theory the .com shoudl always get more natural traffic.
They probably all look equally appetizing to Microsoft's legal department Kidding "themes" seems like a more common search term than "skins" or "designs" in the context of software.
Get the .com domain and then use off-page strategies to boost your rankings for other powerful keywords like Windows 7 skins and Windows 7 themes. Good Luck
the .com is most professional but unless your are selling the domain it will make little to no difference to the search engines which you use. I think it is a lot rumor about the keyword in the URL making all that much difference. Go with what you like.
IMO I would use the .com BUT I would not suggest using the "windows7" keyword in the domain because of possible trademark issues.