Themes and Skins depend on Cascading Style Sheets (CSS) and HTML / XHTML, but Themes and Skins don't fully support CSS - now that's a paradox.
HTML, XHTML, XML, and other related Markup Languages can all be manipulated through CSS - ASP.NET generates most of these languages. If your not yet familair with ASP.NET 2.0 Themes and Skins: Skins give developers and designers a little more precision to manipulate server side controls like datagrids, and calendars, but Skins fundamentally depend on CSS. Themes allow us to group our Skins, CSS files, graphics / images, and so on into logical groups or directories, but Themes really depend on CSS through Skins and CSS (.css) file references.

Both ASP.NET Themes and Skins are directly dependent on Cascading Style Sheets (CSS). However; Themes and Skins prohibit a good deal of CSS's functionality - I'll discuss this is my next post. CSS has been with us for over a decade, it's a pretty powerful display mechanism for web applications. CSS can manipulate HTML, XHTML, XML, SVG, XUL and other common Markup Languages.

I've come up with a number Theme and Skin related questions that I have yet to answer.
 
Questions:

If Themes and Skins rely on CSS why not just use CSS?
Answer: Default Skins are good, they provide hooks (class and ID selectors) for defining CSS. Themes are OK they let us logically group images and design related files, but fall short in a number of areas and are not recommended for large web applications.

If it's possible to accomplish all that Skins and Themes offers through CSS why should I make the switch?
Answer: Skins can make life easier for your web designers, when used to hook into CSS classes and ID selectors.

CSS is a mature, robust language, it's been around for over 10 years so more designers / developers are familiar with it, why should I switch to a a technology that is unfamiliar to most of the people that will be maintaining the application?
Answer: It's not a clear case of CSS vs Themes since you really can't have Themes without CSS. If used properly Themes can augment CSS.

Are Themes and Skins really scalable for a large enterprise level web application?
Answer: Not really, having a seperate server or Content Delivery Network for site resources like images, stylesheets, etc... is probably the best approach. Themes and Skins do prove uesful for small web applications.