How do I change the login screen?

Open config.php in Notepad and search for login configuration:

# Login configuration.
# Set to false if you want to login with your username instead
# of your email address.

#$login_email = true;
$login_email = false;

Change $login_email in true, like this:

# Login configuration.
# Set to false if you want to login with your username instead
# of your email address.

$login_email = true;
#$login_email = false;

Save the results.

Next time you login, Horizon will ask for your username.