Have a look at these links below.
http://www.gnutellaforums.com/limewi...ing-skins.html
http://www.gnutellaforums.com/limewi...tml#post317284
http://www.gnutellaforums.com/322962-post2.html

Those links refer to LimeWire but it's basically the same for FrostWire - all you have to remember is that instead of .lwpt files for LW, they'll be .fwpt for FW.
And when looking for your themes folder, instead of looking inside a folder named LimeWire, you'll be looking inside a folder named FrostWire