When you make any game fullscreen, the minimise, restore and close buttons stop working. Just letting you know before the jam ends.
Viewing post in GodotOS jam comments
This is actually a bug on itch's end because the buttons on the top right of the page might block the mouse input.
You can try fullscreening GodotOS itself (F11) or zooming out your browser and you should be able to click the buttons.
I reported it yesterday: https://github.com/itchio/itch.io/issues/1536