We can add custom CSS in the from the Store ⟶ Configuration ⟶ Magediary ⟶ Store Maintenance Lite ⟶ Content ⟶ Custom CSS field. Reference guide: https://magediary.com/docs/store-maintenance-lite.html#custom-css
The default maintenance page content can be updated from the Store ⟶ Configuration ⟶ Magediary ⟶ Store Maintenance Lite ⟶ Content ⟶ Maintenance page content Reference ...