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.