Turns out, there was a bit of a caching problem! I had my HTML load /theme.css, which was a real file, so if not loading the light or dark theme, a file was hit, and it had an etag and last-modified, and all that. So when switching to a light or dark theme, the browser would use the cached /theme.css instead of trying to load it again, and getting redirected. Changed the setup so /theme.css doesn't exist anymore, there's /theme-system.css instead, and /theme.css is thus always a redirect. The redirect also sets a query param to force the browser to load the page again too, and re-render it. Now it seems to work correctly.