You can already easily change mIRC to look the way you want. Other than getting someone to make you a theme, what are you looking for that isn't already easily done? Try searching for MTS themes and you'll probably find a dark theme already made that will work fine in the current version of mIRC. And even if you can't, it's very easy to adjust colors and such. It's also easy to adjust the toolbar icons if you want to do that, though it's less straightforward.
Yes, mIRC uses MTS themes. Just do a search for MTS themes and you'll find a lot of them. Some will be good and many will be bad, but that's true for user themes for any product.
I should clarify that mIRC doesn't support the themes directly, but it uses them if you include an engine that is easily found. Support for the engine will vary. As TRT said, the engine being built into mIRC may never happen because it's fairly easily to do it already depending on what you want to change.
Or make your own, not that hard, fun and you will lern somthing during the way. I made mine so i can edit colors of all texts true a dialog. I also made everything editable so i can change everything to what i want it to be