Add Custom CSS to WordPress Admin Area

To do this, add the following code to your functions.php file and change the CSS as needed. If you want to know the right way to add assets (CSS and JS files) to the WordPress admin, take a look at the post Loading scripts correctly in the WordPress admin.

function my_custom_fonts() {
    echo '<style>
    body {
      font-size: 12px;
    } 
  </style>';
}

add_action('admin_head', 'my_custom_fonts');