I had the old forum on dark mode through Chrome as well.
I've found that the DarkReader extension severely slows down my browser, sometimes to the point of crashing, especially if I open a lot of tabs (such as when browsing the forum). Plus, it doesn't always work perfectly (as one can see with those white icons). An integrated dark theme works best in any case.
Is it possible to make the "About us" section (the greyed out section) smaller in height? It seems to be taking up a bit too much space.
Yep. The part which has the About us, quick navigation and user menu.Do you mean in the footer, at the bottom of the page?