Add css code in admin_enqueue_scripts

Question

If you’re using the admin_enqueue_scripts hook, use wp_enqueue_script() to enqueue a CSS file with the styles you want to apply.

If you want to output a <style></style> element use the admin_head hook to output it between the <head></head> tags:

function custom_css_stuff() {
    ?>

    <style type="text/css">
        body { color: red; }
    </style>

    <?php
}
add_action( 'admin_head', 'custom_css_stuff' );