Let users customize your website with their favorite color scheme! Your site has a dark mode? That's cute. Mine has ten different themes now, and they're all named after Mario Kart race tracks.
I love the idea of this sort of color theme switcher. Reminiscent of the sort of functionality built into TiddlyWiki. I suspect that some of the code built into WordPress’ Customizer could be repurposed to give people the ability to do this in the WordPress world.