Firstly, when I first read this (up to 2nd sentence), I thought you were talking about a product called "Skins", or talking about human skin. My bad. Well, you have to excuse that I don't speak English.
Anyhow, back to your question (which I assume you were referring to browser skin/theme). Personally, I don't mind things like donationware, as long as "donation" is not compulsory. I don't see an issue that at bottom of the page (or somewhere) that includes a donation link. Users are not forced to donate, and if users like it and feel generous, the donation method is straightforward. I think that is much better than paid version (which users must pay before trying the skin/theme out).
As for those paid version, I don't think it would work that well in the browser skin/theme. However, I do pay for a few drawing/graphic/vector arts (.png or .svg) since I know I couldn't have made those myself, and the free ones are just not that suitable.
UserAgent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.7; en-US; rv:18.104.22.168) Gecko/20120128 AlexaToolbar/alxf-2.14 Firefox/3.6.26