Windows: We've detailed the tried and true method of patching Windows to allow custom visual styles, but if you'd rather not tweak files in your system's guts, the tiny UxStyle background app simply opens the door for any custom theme.
There's not much more to say about this app, really. Grab the package, install either the x86 or 64-bit version, depending on your setup, and after it installs (and explains why it "phones home" every hour, for testing purposes), you'll see UnsignedThemeSvc.exe running on your system, taking less than 2MB of active memory. While it's running, you can load in any themes you'd like through Windows' own theme manager, using all the steps detailed in our guide happening after the part about patching uxtheme.dll.
UxStyle is still a beta app, and may not fly with every Windows setup out there, but it has been reported to work on XP, Vista, and 7 systems. It's a free download for Windows systems only.