CSS of the admin panel

One way is to enqueue a style.css for the admin panel.

Create a file to say custom-css.css and add css to it and then enqueue it to the admin by putting the below in the theme’s function.php file.

function admin_enqueue() {
    wp_enqueue_style( 'custom_css', content_url() . '/themes/theme-name/custom.css');
    //Please replace the path with the correct path to the file in your theme 
}
add_action( 'admin_enqueue_scripts', 'admin_enqueue' );