In the editor window the vertical bar showing the code folding has false background color behaviour. Insted of showing just the coloured background, there is a black grid overlay to the set colour (image linked).
It happens on Kubuntu 20.4 with a high dpi display (15.6" with 3840 x 2160 px) if the display global scaling is set to more than 100% (in my case tested at 200% and 250%).
Diff: