This is definitely a difficult problem - particularly if you want to somehow weave lots of custom styled hypertext together. But I feel that not a lot has been attempted yet.
One thing that’s interesting to me is to see how many people customize their Tiddlywikis. It seems like many static site generators have ‘good enough’ styles. (It also may not be clear how to tweak the default themes.) However, Tiddlywiki has such a bland basic theme that one immediately wants to go beyond it. So perhaps having poor defaults is good for a platform.
I also think whostyles - the concept for this came from Sphygmus’ wiki - show some promise. At least let people style some part of their post colors, fonts, outlines. This could be done successfully on Reddit - though perhaps they would limit the color palettes and font choices if they didn’t want readability to suffer.
Not really sure the answer to all of this. But thanks for the thoughtful comment!