As mentioned above.
This is not Node RED, this is your browser, advising you're not using a secure connection to Node RED.
See here to enable HTTPS in Node RED
https://nodered.org/docs/user-guide/runtime/securing-node-red
As far as changing the logo on the login page, see here.
If you wish to go beyond just updating the logo.
https://nodered.org/docs/api/ui/themes/
EDIT
Just realised this is specific to Dashboard, but I'll leave it here anyway. 
The login prompt you are referring to, cannot be customised, as its part of the browser you're using.
