How to customize the login screen?

You can configure the theme options in the [hosted login page] (https://manage.auth0.com/#/login_page), in Auth0Lock, as:

var lock = new Auth0Lock(config.clientID, config.auth0Domain, {
(...)
theme: {
         authButtons: {
           "testConnection": {
             displayName: "Test Conn",
             primaryColor: "#b7b7b7",
             foregroundColor: "#000000",
             icon: "http://icons.veryicon.com/ico/Emoticon/Chat/away%20girl.ico"
           },
           "testConnection2": {
             primaryColor: "#000000",
             foregroundColor: "#ffffff",
           }
         }
       }
(...)
});