Calm Dark is installed.

Open Steam and start Football Manager 27. The first start takes a minute or two longer while the loader prepares itself (once, and again after an FM update).

Then open Preferences and pick Calm Dark from the skin list at the bottom of the window.

If FM needs double clicks on that first start, quit it and start it again; it does not come back.

To uninstall: ~/Applications/ImmersiveFM/Uninstall Calm Dark.command (double-click it). The README is in the same folder.

If Steam's launch option could not be set, see ~/Library/Logs/CalmDark-install.log.