No need for extensions or any third-party software to make Youtube always play video in Theater mode.
Go to https://www.youtube.com
Open dev tools of the browser you are using (Press F12 or Right-click -> Inspect)
Select Console tab
Paste below code and Enter:
document.cookie = 'wide=1; expires='+new Date('3099').toUTCString()+'; path=/';
how does one undo this in chrome on macos sonoma?
It is so easy Jeff. Open youtube and press F12 or Right-click -> Inspect
That will open the Inspect window. Go to Application tab and click to Cookies section. Choose youtube.com cookies by clicking on it. You will see the row named “wide”. Just remove that row. So you will get rid of that.