If you add ?disableCustomCSS at the end of any URL (for example, https://remnote.com/notes?disableCustomCSS), that will turn off loading of custom CSS for that session.
You can also press Ctrl+Alt+Shift+C (or Mod+Opt+Shift+C if you are using a Mac) to toggle all Custom CSS Snippets.
This can come in handy if you are editing custom CSS and accidentally break the formatting so badly that you can't get make your way back to the custom CSS page to change it again.
See more documentation at the bottom of the custom CSS docs page.