I’m using DarkReader in Firefox, so that may be the difference. I have played around with it, and I found that turning DarkReader on and off, and toggling the site enable checkbox off and on again will get it to work. For most sites, it just works, so I was thinking there was something unusual here. It is different behavior, but can be made to work. Thanks!
Regarding the original thread I posted to, it would be nice to have a site-wide theme preference that includes a dark mode.