Gnome Tweak Tools & tar.xz, tar.bz

Good day all,

I'm trying to add new themes and icons to F18 via the gnome tweak tool. Most of the themes I have no problem installing as they are all .zip extensions, however one such theme is only available in .tar.xz or .tar.bz extensions. Gnome tweak tool simply doesn't recognize it and can't load it. What could I do to convert the file over to .zip?

Thanks much