Custom Admin Styles in WordPress

functions.php

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