keynote 12.1 - the presenter screen is dark when I start the presentation. It looks fine on editing the presenter screen.

Never heard of this problem but try these two things

1 - create a new Mac user account then test Keynote: Set up users, guests and groups on Mac – Apple Support (UK)

2 - reset Keynote: complete all of the following steps in the order listed:

• ensure you have a current backup of all your user files, saved to a separate drive
• close all applications
• uninstall Keynote; for this procedure to work, an application removal tool must be used, App Delete has a free trial and works for this procedure. It can be downloaded from the developer’s website here:  App Delete
• empty the Trash (Finder > Empty Trash)
• restart the Mac (Apple Menu > Restart);  Immediately after the Apple startup sound, press the shift key on the keyboard, release shift when the Apple icon appears. Start up will take longer than usual as the Mac is performing a software repair of the start up drive
• After the Mac is fully powered up, restart the Mac normally:   Apple Menu > Restart
• reinstall Keynote from the Mac App Store Application
• Restart the Mac again