How to apply a CSS file to my admin pages?

Try this code by adding in function.php file.

function Newcustom_script_enqueue() {
  wp_enqueue_style( 'customstyle', get_template_directory_uri() . '/newtheme.css', array(),'1.0.0', 'all' );
}
add_action('wp_enqueue_scripts', 'Newcustom_script_enqueue');