Great work, @Litalino. I'm being an early adopter of your extension. I really consider it very helpful. In any case, I think it only works with sites with a white header, which fully breaks my site design. Otherwise, it produces the effect below:
I think it would be very valuable to extend it also for Colored headers, as this is a standard feature in Flarum which can be activated from Appearance > Colored Header in the admin menu:
I hope it is easy to reproduce in your own setup. Unfortunately, I need to disable the extension right away in my own environment, as I have already got some user complains.