Dark mode does work on forums but you have to activate it inside the forum.
- Click on your avatar top right (your account button) >
- preferences >
- style >
- UI,X Dark.
- Then hit save
- then refresh the web page.
To be clear this is in browser, not the app, that may be different (I am still learning too) but I see same dark mode there that I set up first on the browser.
Also, your use the same path/menu to set the font bigger just below the dark mode.
You can also disable the video player there. This loses some content/functionality, but it also removes roughly 1/3 of the bright shiny object on the forum page. You can restart the video player at any time with a button that will appear top right on any forum page.