Which is why I suggested pairing it with user settings so that it would be easier since they would just need to toggle. If you could first reach a place where everyone was with it on the formatting side then you could move forward. As for covering the text colors in relations to the many theme variants you could possible address that with some css/js trickery that was based on their preference but that insured enough contrast to based variant rule..... I'm not much of a front-end guy so take that with a grain of salt