A known Windows Insider bug has entered the Windows 11 release: Task Manager will show wrong colours (on my stock Dell Windows 11 installs, see below, it will be dark on dark) under certain custom Windows colour settings (“Choose your mode” set to “Custom”).
Edit 20221214: it got fixed
OS Build 22621.963 [Wayback/A] KB5021255 fixed this on Patch Tuesday (which arrived on my machines later than my original publication), see [Wayback/Archive] December 13, 2022—KB5021255 (OS Build 22621.963) – Microsoft Support