More modern? I think 'modern' is an entirely subjective concept. And for all that I've said already about the Avast GUI, it isn't horrible by any stretch of the imagination, even though I'd still much prefer my system 'theme' to apply to the software I am using, but that is my particular preference. However, that said, I do think alternative GUI colour schemes would be nice touch actually...although I know now that there won't be a 'default system' setting ever because Igor has already stated that there is no way to 'unskin' Avast, well not without considerable redevelopment, which is a shame but not exactly the end of the world.
I have to say that as is, even considering visual accessibility, the current Avast GUI seems to cover most bases (needs more config options still, but that's something else entirely ;-) ). And even if you are colour blind I don't think the GUI will do your head in too much, or prevent you from accessing the features. To test this I snapped the Avast GUI and uploaded it here:
http://www.colblindor.com/coblis-color-blindness-simulator/Seems pretty much accessible no matter what colour blindness you happen to suffer from...but alternative colour schemes would still be nice, even if not really essential.