About "dark" theme.
The OS, Browser, and many other apps are also equipped with the 'Dark' mode (theme) Option.
In order to switch this add-on to Dark mode, it is necessary to incorporate CSS code (https://github.com/brief-rss/brief/issues/326) into "Customize Brief Look" of Option, and it takes a little effort. And many end users don't understand how to do it.
"Dark" mode has high needs, so I think it would be better to add it to Option and make it easier to switch.
+1
To be honest this should probably just be wrapping the dark-mode CSS in @media (prefers-color-scheme: dark) { }, and then let the various system and browser toggles for light/dark modes do their thing. This can be used both in the customized CSS and the default CSS.