I spend a lot of time on the computer, using a lot of software. This makes me acutely aware when things don’t quite work right. A button that renders incorrectly. Inconsistent padding. Unthemed dialogs. Even the best themes out there had small bugs that annoyed me. I care deeply about a smooth, consistent desktop experience.
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.