I use the above "Dark Background & Light Text" Firefox extension on my desktop, & Kiwi Browser on my Android device — Kiwi is built on top of Chrome, and it has a dark mode — so all websites I visit are dark. It's fantastic.
Alas, it doesn't work on Android
Edit: I mean it works but not for ESO forums, somehow on my phone
Oh for your mobile phone. AFAIK there is no way to add extensions on mobile chrome without downloading sketchy 3rd party apps. I think you'll have to use a different browser on your phone.