In the case of Brave and Vivaldi, they add their own undesirable parts (Brave adds crypto bullshit and Vivaldi is closed-source, so $DEITY knows what they’re adding).
Librewolf is open and doesn’t contribute to the Chromium monoculture; so it’s the best option
I used to be really into theming. But now, the default Breeze and Adwaita look good enough that I haven’t bothered wanting to change them in a couple years.
That and thmes always appeared to be some degree of “broken” that I just don’t bother anymore.
I do always change the cursor to the black Adwaita one, even on KDE. It just feels right to me.
When I did still use themes, Numix, Arc Dark, and whatever “flat” themes that I could find were my favorites.