Menu and user interface colors broken on Linux Mint

I am using Linux Mint 21 and have just installed Qt Creator 9 via the maintenance tool. In general, I use the interface theme "Flat Light" so far. Choosing this, the menus are displayed in black font color on a black background (background should actually be light). The background color of the sidebars is also black, instead of white. But even if other themes are chosen, the colors are sometimes mixed up.