Again, like every time I mention it, RSS readers with embedded browsers won’t support extensions.
You don't need extensions.
There are many RSS readers with inbuilt dark theme / night mode.
Even if you prefer self-hosted RSS readers, you can find RSS readers (FOSS, not Premium/Freemium) on GitHub with such feature.
https://alternativeto.net/software/feedly/?feature=night-mod...
https://duckduckgo.com/?q=github+rss+reader+self+hosted+dark...
This might be a goofy solution but it works: I'm on mac and used BetterDisplay to create a virtual inverted monitor that I throw things to.
You don't need extensions.
There are many RSS readers with inbuilt dark theme / night mode.
Even if you prefer self-hosted RSS readers, you can find RSS readers (FOSS, not Premium/Freemium) on GitHub with such feature.
https://alternativeto.net/software/feedly/?feature=night-mod...
https://duckduckgo.com/?q=github+rss+reader+self+hosted+dark...