Well it’s officially Super Tuesday so 6.5 is live so why not celebrate by making a custom theme? Microsoft has set up a simple site to choose your colors, choose or upload your wallpaper and then get a link to your theme to install. They only list Pure and Imagio as supported phones which are both WVGA phones so at a minimum it supports WVGA phones and I presume other resolutions will work just fine except the wallpaper may not size right (or you can just manually set a wallpaper:)).
If you want more sophisticated themes just cut to the chase and go to Mobile Matt’s site (check out the new TP2 FuzeBerry 3 theme!)…sorry but MS can’t compete with that:)