So, if I’m reading this right it’s basically just a 17 paragraph essay that boils down to, “Sorry we suck at CSS and it took us a decade to finally get around to rooting out all the random shit from 2014 that was hard-coded to display as rgb(0,0,0) or whatever, which was a capability that in retrospect we really shouldn’t have handed out like candy?”

The TV Tropes wiki has managed to have a built in dark mode for at least the last 7 years. TV Tropes. Come on, guys.

I’m baffled by the section about “making a shortcut that darkens all the colors on the page.” I’m positive that’s the intent of that entire blurb, to dazzle people with bullshit in the hopes that they won’t ask Hard Questions, because no competent designer would ever try such a thing. It is a self-evidently moronic idea. You don’t fuck with elements you didn’t create and don’t control, like images and color swatches.

There are only really two viable possibilities, here:

1. If arbitrary user definable, hard-coded colors in content are permissible, you’ll have to accept the fact that the cards will fall where they may and some instances will inherently be suboptimal in either light or dark modes, or…
2. Accept that you won’t allow users to hard-code colors into anything outside of specific elements where that usage is valid, so users will just have to suck it up and pick from a list of preapproved color combinations with light and dark mode renditions.