Hello!
Could you clarify what version of Superset do you use?
1-2) I tried to reproduce with 4.0.1 version like in guide here: Getting Started with Superset :: Jmix Documentation but I did not see a notification. Not sure, but probably by “refresh” it means getting new guest token for this dashboard. In this case, new guest token will be fetch every 5 minutes as configured in Superset GUEST_TOKEN_JWT_EXP_SECONDS property.
3 ) I guess it can be configured by role permissions that is assign to Superset user. It is better to check Superset docs and tutorials.
You can customize CSS for specific dashboard. When you edit a dashboard there is “Edit CSS” button in the settings. Unfortunately Superset does not provide full template for customizing dashboard, so you should check the DOM elements and CSS class names in dev-tools. In the example below, the Title has a green color.