It has happened to me a couple of times already after upgrading to macOS Monterey...
It looks like Xcode and iOS Simulator can get into a state where Command-Shift-A (⌘⇧A)
keyboard shortcut for switching between light and dark mode in Simulator just stops
working. It is registered by the menu just fine, but it doesn't do anything... I've
found one way to fix it:
$ sudo rm -rf /Library/Developer/PrivateFrameworks $ open /Applications/Xcode.app/Contents/Resources/Packages/XcodeSystemResources.pkg $ xed .
This re-installs Xcode's private bits, and things start working again...