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.