A compositor (like compton) is ***required*** for this theme to work as intended.
This is in part due to the fact that it makes the top bar look less bland, and in part due to the fact that your titlebar corners will look weird otherwise (the corners won't be properly cut).
You may, of course, dismiss using a compositor, but don't tell me about not exactly round corners being a "bug" in this theme afterwards - it's the best you can get with X11.
