While not an option by mmo-c directly, you can get the browseraddon Stylish (
Firefox,
Chrome,
Safari,
Opera [only used it for FF and Chrome so far, but it works like a charm])
and then install this skin for mmo-c:
https://userstyles.org/styles/76166/dark-champion
Also works for any other sites and other colours schemes of course, e.g. dark google, youtube, you could probably make your twitter pink or whatnot.
Just look for it in the search bar.
There are also some other mmo-c themes there, so you might like those even better than the one I've suggested and have been happily using since a few years. :3
€:
As for avatars and signatures, Winter Blossom is a perfect example (thanks mate).
As you can see, as long as you or others use a transparent background it's all fine, if you use EFEEDC or similar for it, it of course won't blend in anymore.
That never really bothered me personally, but it's something to be aware of.