I’m a big fan of modularising functions and only loading resources when needed. This snippet should be in your functions.php file and will only load an include file if a visitor is on the login or registration page. You can then squirrel your functions that customise the login experience or style it with CSS away here.
if ( in_array( $GLOBALS['pagenow'], array( 'wp-login.php', 'wp-register.php' ) ) ) {
require_once locate_template( 'INSERT_FILENAME_HERE.php' );
}
WordPress – or should I say JetPack – has a lovely habit of loading a large amount of cruft that you may not want on your page. After the recent WP 4.6.x update I found the custom login page on one of my sites didn’t load properly anymore, because all the custom css was being over-ridden by css files WP was injecting at page load. This function stops this injection:
add_action( 'login_init', function() {
wp_deregister_style( 'login' );
wp_register_style( 'login', "" );
wp_deregister_style( 'buttons' );
wp_register_style( 'buttons', "" );
wp_deregister_style( 'l10n' );
wp_register_style( 'l10n', "" );
wp_deregister_style( 'forms' );
wp_register_style( 'forms', "" );
wp_deregister_style( 'dashicons' );
wp_register_style( 'dashicons', "" );
} );