Hi, I am new with SuperPowers and I do agree with you. This engine is already great and with some support it can become a huge one.
I am still discovering it so I cannot be very helpful for now, but I definitely want to give a hand when I'll feel confortable enough with it.
For the custom themes topic I guess the actual theme is in core/public/styles. I think the easiest way to achieve it is to use SASS. Like that you can expose variables (for the colors for examples), so that you can easily try and create new themes.
The editor should also be customisable depending on the theme, for that I would definitely recommend ace, if it is not already the case.
But I don't think custom themes is the main issue for now...