Experimented a bit more with custom HTML tags today. I have my own presentation tool based on @ratfactor@mastodon.art minslides, I changed the individual slides to a custom tag but I also added in a custom attribute for the slide's colourscheme.

Went from <div class="slide colour_light"> to <slide theme="light">. The attribute can be targetted with [theme="dark"] in CSS and then styled as needed.