Hello Basj,
thanks again for your quick reply, but I think we're talking at crossed purposes. I don't know how to make this more clear, the behaviour in light and dark mode of Groupy is different, whereas the light mode behaviour makes sense, and the dark mode behaviour doesn't.
Again, I have provided a screenshot, once with titlebar accent color on and once off. You can clearly see that the top line disappears in 1a in light mode (because it obviously is not needed when the accent color is displayed in the title bar. If accent colors are off, the line is displayed (which makes sense). But in dark mode, in "1a" the line (plus the weird line on the left of the tab) is displayed, which probably is not intended/doesn't make sense. Which means the behaviour of the UI in Groupy is different in light and dark mode.

Another direct comparision of light and dark modes:
