Thanks for your reply, Timo!
Maybe I misunderstood your solution, but where do I put the HTML file, because all of the skins have their own folder? I think the HTML should be centralized outside of the skins folder?
And how does this centralized HTML "know" which CSS to include, as this has to be a different one depending on the skin?
I really hope this solution can work, but a quick test with using a ASCX who includes a HTML file left me with an empty skin... :(