I think "complex" refers to the various dark patterns used by Windows and Mac/iOS to scare and/or force users that know nothing of computers into using the default browsers.
Can you list some more specifics then? Distro, what the theme should be, which applications are not respecting theme and how, are they GTK 3 or GTK 4, how are they installed (package manager, flatpak, snap, etc)?
Disable all extensions first, then reenable individually.
Flatpak and snaps do not integrate directly into system themes without special care. No idea how snap works, but flatpak requires some 'bridge' theme installed to properly interface with the system theme.
It's very different now. Much more smooth. Of course you still configure the system manually. But following the handbook will get you a working system pretty easily.
Didn't they contribute networking stuff?