Oh, I remember why I installed it. My eyes were hurting and the only way I could use the PC was by turning the screen brightness way down and browsing everything in dark mode.
You're confusing two different confusing systems, let me demistify this for you.
1. Native dark mode that the page can automatically infer from your browser settings (light/dark/auto) -- this XenForo theme does use it
2. Injected dark mode using addons like Dark Reader, by default only active on websites without the former