I finally figured out how to change the skin. Basically, click on and then get more skins.
When you've found a skin you want, remember the name.
Go into limewire and search 'All Files' for that skin.

A few different ones should come up, wait for it to finish and download the one at the top as its most likely to actually be the skin.

After its downloaded a pop-up should come up asking you if you want to apply the new skin you've just downloaded, just click apply/yes.

And your new skin will be applied.

Good luck x