Try out the program I linked to. Not only does it install a hacked version of uxtheme.dll, it backs up the original uxtheme.dll file so that if you find problems, you can simply run the program again and restore the orginal. I've used the hack and installed non-Microsoft themes. Give it a go, find some themes you like (same website) and see what happens.
Microsoft originally intended the themes to be something they provided. But as you can tell, they decided that it wasn't lucrative, so they didn't provide for many alternate themes (aside from the Plus! pack for Windows XP). You can buy good programs like (Stardock
, but this is one of the ways that you can get themes free and without adding on a program to suck up CPU cycles.
Plus, if it does cause problems (really, it won't...), you've got tens (hundreds?) of guys who are here willing to help!
A good place, by the way, to go for themes is ThemeXP
. Check it out, whatever hack/program you decide to use.
Also, it seems that the UXtheme Multi-Patcher has been updated recently. Check out this link
for the latest version.