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' );