for this you can have two options
1- use widget like sign in, sign out for custom login page
2- create a new page and select that page as login it will be the same by default page and after that in navigation you can add that page as login page
inside your app's directory, inside the theme folder you will find the html file and by change that and re-running your app will reflect your changes
you can change the security to production automatically it will take login.html
Hope this will help you
Regards,
Geeta