It only makes gtk theming slightly less of a pain in the ass, but you can use gtk’s inspector (enabled thru a gsettings terminal command and then ctrl+shift+d in the app or thru an environment variable) to find the classes, names, and object types for whatever you’re trying to theme.

Honestly Gtk theming has extremely poor documentation and you need to look closely at existing themes to figure out all the CSS selectors you need to define. It is definitely possible, but can require some trial and error.

Not all themes are created equal, but the titlebar is actually one of the easier things to theme.

That being said, it really depends on your DE, and if the apps you are using are Qt or Gtk 3/4.

I really wish they had better guides online, but that was the main reason I spent time working on Celestial. To finally have the consistency i wanted, and untangle the theming mess.

As far as i understand it (never made one, only modified), it's like creating a global usercss for the web (that i did), except with imports and symlinks. You have some generally used class names that fit most of the apps, except that one tool and then some. Then there's gtk 2.0, 3.0, 3.1, 3.1/4⁴, 4.0 and the window theme, which is (luckily still) separate.

Writing a custom GTK3 theme for my own use a couple of years back was an extremely painful process. There's no list anywhere of the possible themable element types (I had to go through the actual source code to compile one) or the possible nonstandard options (never did manage to compile that list). I haven't had to look at GTK4 yet, but I doubt it's any better.

(As for how people use dark themes: put borders on things if you need to, and/or use hover options to distinguish what the active element is.)