This is correct. Unless it involves personal data and/or tracking the user somehow, then GDPR isn't relevant.
Example: If you're storing lightOrDarkTheme in a cookie/localStorage, then there is no need to try to follow any directives, nor are you required to inform the user about that you're storing the preference.
Example: If you're storing lightOrDarkTheme in a cookie/localStorage, then there is no need to try to follow any directives, nor are you required to inform the user about that you're storing the preference.