Use default header color values
As a long time user of your extension, now that I'm on GNOME Shell 40, first thing I noticed was the headerbar bug showing a pure black color. I had to check the extension screenshots once again to be aware that it was intended.
Whilst this is your project and you decide how it is any way you like, the pure black header looks like a render bug. I think adding a gradient or some other color would not cause a new user to think there's something wrong with their system.