Personally I would just use the true false field because this
$value = 'dark'; // default 0/false value
if (get_field('field_name')) {
$value = 'light'; // 1/true value
}
Is much easier than creating a new field type.
You could also add new field settings to the existing true false field https://www.advancedcustomfields.com/resources/adding-custom-settings-fields/ and then add a filter as well to the format field hook. Then you could set the return values when creating the field and return the values you want.
Just some more ideas for achieving what your looking for.
