That's a hard problem to solve, since there are many ways of styling Electron - Bootstrap/Bulma/Material Design/custom stuff. So it's hard to just override CSS rules.

On the other hand, it's the kind of problem perfectly suited for Machine Learning - just take the whole screenshot/image of the app and use a stylistic ML to change it - make it Dark Theme/whatever...

I suspect in 10 years OSes will support ML-based automatic themeing/customization of all apps, regardless of what they are written in.