Load/Enqueue custom CSS for WP Admin Dashboard

admin.php

<?php
// Update CSS within Admin Dashboard
function admin_style() {
	wp_enqueue_style( 'admin-styles', get_template_directory_uri() . '/css/admin.min.css' );
	//change filename and/or url as required
}
add_action( 'admin_enqueue_scripts', 'admin_style' ); //load on admin pages
add_action('login_enqueue_scripts', 'admin_style'); //load on login page
?>