The preference is stored in a cookie-like way using the browser's localstorage system. As you might guess, this is not stored in a private browser mode. It makes me wonder why CryptPad seems to store so much info in localstorage, such as your username, login token, timestamp of last visit, premium status, and a ton of CSS. Also, how easy it would be to compromise a user's CryptPad account on getting access to that localstorage, assuming that the user didn't previously explicitly log out.