eh... XML… CSS potato/patoto... not terribly different to this marginally educated monkey. I was able to figure out what/how to change what I needed to in the CSS for the TI templates with a little trial and error. They are similar enough to me I'd forgotten that the main TI file was not an XML.
That said, I have not futzed around with HTML or JavaScript at all, so there is that. But, the simple built-in MC TI templates do not use JavaScript. They do use an HTML File/Image that has embedded text - not sure how you edit that. I imagine with a little googling though, it can be sussed out.
Videlous - don't let ET and marko scare you away from trying to make a dark custom TI of your own!

If you have the time and desire to tackle it, I'm sure you'll succeed.