While almost any context menu shows a dark background color, e.g. lister file context menu, the context menu of the folder tabs does not
I don't remember where to set up the color for file context menu or at least I could not find it in preferences Display->Colors and Fonts. Would you be so kind to point me into the right direction?
And second question: is there an option to change the color of the tab context menu?
That's a standard menu drawn by Windows. (A lot of the other ones are custom menus we draw entirely ourselves, but we use standard menus in places where we don't need any of our extra functionality.)
Windows doesn't currently provide a supported API to switch apps or windows into Dark Mode. There is an API, but it's undocumented and still changing, and regularly breaking things that try to reverse engineer it. Dark Mode is still a mess in Windows at the moment. (A mess that shouldn't exist if Microsoft hadn't already made a mess of the Visual Styles system!)
We are keeping an eye on this, and hoping Microsoft stabilise and publish the API, but it is not ready for use by third parties yet.