Step 1: Create Your CSS File
You can place the CSS file wherever you’d like; I’ve chosen to place the CSS file within my theme. My admin CSS file looks like:

.wp-admin .comment pre {
  background: pink; /* they forgot the language! */
  padding: 6px 10px;
  font-size: 16px;
  border: 1px solid #ccc;

.wp-admin .comment pre[class] {
  background: #fff; /* language (likely) there */

The CSS above makes

 tags more visible.  It also will make any PRE element without a class more apparent, teling me I need to fix it by adding the language as a className.

Step 2:  Add Your CSS to WordPress Admin in functions.php
WordPress uses an add_action type of admin_enqueue_scripts for adding stylesheets anywhere within WordPress:
// Update CSS within in Admin
function admin_style() {
  wp_enqueue_style('admin-styles', get_template_directory_uri().'/admin.css');
add_action('admin_enqueue_scripts', 'admin_style');

get_template_directory_uri provides the path to your current theme, you simply need to add the filename to the end of the path. By David Walsh on October 7, 2015