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.

Absolutely, the ability to actually customize your computer to the smallest detail is what brings the uniqueness to our setups.