This is the same problem I faced recently when I wanted to quickly create a presentation for a talk at UNIR with a dark color scheme (for A paper about Typst, written in Typst, and with an official Typst-based template in the IJIMAI journal). I basically reversed the colors, but, IIRC, I also had to change some colors etc. in the template to make it actually work. I think there was an issue with a header? I don’t have a diff of the changes in that file in the repo. I mainly had to create a plain-slide function, so the rest is mostly unchanged.
Yeah, the header text uses the same color as the slide background, which means you can’t have opposite/different text color in the header.
So, I think not all templates can have dark theme, especially if they use, e.g., hard-coded colors. You’d have to manually test/check each theme though, I guess.