Adding Custom CSS to the WordPress Admin Area

For this, we will use the admin_head hook.

Create a function in your functions.php file, for example:

function my_custom_fonts() {
  echo '<style>
    body {
      font-family: "Manrope";
      font-size: 16px;
    } 
  </style>';
}
add_action('admin_head', 'my_custom_fonts');

Done!

For more information about hook, please use this link.