How to sync the theme with the system appearance?
NotePlan already observes when the system appearance changes and adapts the theme automatically. If this is not happening for you, you have probably changed the theme.
When your device is in dark mode and you select a dark theme, NotePlan will "remember" that this theme should be used during the dark mode. Vice versa, if you choose a theme while your system is in light mode, NotePlan will use this theme every time your system changes into light mode.
So, to fix this, go into the preferences and select the respective theme fitting to the current system appearance. Change the appearance and do the same again, but choosing another theme.