In Windows dark mode, yellow works better for me than red; red's okay in Light mode. See attachment.
In dark mode I sometimes find the default colour (blue) hard to see. The MS response would be: "Turn on High contrast". It's notable that MS have switched to bright white for Dark mode, and pitch black in Light mode in some of their software (Settings, Office, Edge etc).
If I undock a child window (e.g. Book Browser, Clips etc) it will never get keyboard focus via a Tab - those child windows will only get focus via a Tab if they are docked.
BR