But here's a larger one: coloured text. A common practice, that I'd be flabbergasted if you hadn't seen, is people using coloured text to differentiate between different character's dialogue. Colours that are easy to read on one background would be next to invisible on another. Oh, you could figure out what colours are legible across all backgrounds and limit yourself exclusively to those, but it'd be a headache and an unnecessary extra layer of complication for something that's supposed to reduce confusion.
Easiest way to deal with color fonts is to simply find the troublesome fonts on lighter/darker themes, and via CSS assign backgrounds to the colors so that they're legible. Much like Mahz's styling for inline coding used to be. It still can be a bit of an eyesore, but it won't make text invisible or make someone go blind trying to read it. Besides, it depends on the color scheme, the more white you go the harder the colors become. Sticking to a lighter grey or beige helps this effect.
As far as fantasy goes (too many posts to quote directly), why not simply generalize the tooltips and add a Fantasy tip? There's definite contention about what high fantasy is, as such I disagree with HeySeuss' (and Wikipedia's) inclusion of Harry Potter as high fantasy. For me, high fantasy is generally a world where humans are not the main race, so Tolkein's Middle Earth would be a perfect example. Narnia fits well, but Harry Potter still has humankind as a the main race, creatures and human-like monsters are still in the minority. But that's why I think fandom tags, once user-made tags are implemented, will be far more useful for eliminating those arguments. People who create their own fantasy worlds are entitled to their own definitions (just like how GIF is pronounced with a soft G :P).