Add Custom CSS to the WordPress Admin https://jonbellah.com/add-custom-css-wordpress-admin/

wp_admin_inline_css.php

function my_custom_admin_styles() {
   ?>
        <style type="text/css">
          .your-custom-css {
               height: 300px;
               overflow: scroll;
               border: 1px solid #ddd;
               padding: 15px;
           }
         </style>
    <?php
}
add_action('admin_head', 'my_custom_admin_styles');