Style classic editor buttons

Question

You can add CSS to the admin area of WordPress using the [admin_head][1] hook

add_action('admin_head', 'my_custom_admin_styles');

function my_custom_admin_styles() {
  echo '<style>
    #qt_content_center {
         color: red;
    }
  </style>';
}

Note: Alternatively, you could add a stylesheet using the same function.

Edit: Adding custom stylesheet to WP Admin.

add_action( 'admin_enqueue_scripts', 'load_admin_styles' );

function load_admin_styles() {
    wp_enqueue_style( 'admin-styles', get_template_directory_uri() . '/admin-styles.css', false, '1.0.0' );
}