What is a Skin anyways?
Skinning were around pre-.NET, but it was not known as Skins.
When you color a text in your page, you are actually skinning the text. The web gurus abstracted this away into what later came to be known as Styles. Then came CSS (Cascading Stytle Sheets), where you could collect all the styles that you applied to your pages, into one single file. Then there came standards for CSS. This collecting of the styles to one file(.CSS file) was a revolution indeed.
You can relate a style applied to a control in your web page as a Skin and the .CSS file as the Theme of your site.
Themes and Skins - Part 2