I’ve been using Editorial on iOS for .taskpaper together with FoldingText. One of the things I like about Editorial is the ability to assign tags to color labels (it’s actually handled this way and not the other way around): in the option menu, you have a bunch of colors, and you write the tags you want to cause the text background color to change behind them, see Editorial for iOS or here:
It’s a very quick way to get limited (but in my personal usage of .taskpaper sufficient) theming, and it doesn’t require editing .less files or similar.
Anyway, it’d be cool if such a feature eventually came to TaskPaper 3.