I think this would do it
https://github.com/the-allanc/minty-color-titles
It says to put it into the .config/gtk-3.0 folder
But I can’t find a .config folder. I found an /etc/gtk-3.0 folder.
Would that directory work?
Is there a better way to change the title bar color?
In the file manager View menu, check the box for Show Hidden Files (or just press Ctrl+H). Then you should be able to see the .config subfolder. Go into that folder and see if there is already a gtk-3.0 folder. If not, use Create New Folder (it’s in the File menu or right-click menu) to make one with that exact name. Then you can copy the file into it.
Have you tried creating that directory?