Minor issues with "Close Terminal" dialog
The "Close Terminal" dialog has both a "suggested" and a "destructive" action in the headerbar. I'd suggest doing only the latter (i.e. removing the suggested styling on cancel), since having both looks a bit odd.
Also, using "OK" as a button label is always dangerous because it doesn't spell out what will happen, so people blindly click it. A better label could be something like "Close" or "Close Anyway".