Currently the only way to edit the preferences is if you have a design open. Then you can do File > Design Settings. In this Design Settings dialog there are two distinct sections: Application, and Design.
At present the application settings are not editable unless you have a design open (File > Design Settings are grayed out). But application preferences are global and not tied to any particular design so they should be accessible regardless.
This should probably be separated out to either a (general) Preferences, and a separate Design Settings dialog. Or have the Design section grayed out if there's no design open but the Application section is still available. But not being able to access the global Application settings unless there's a project open doesn't make sense.