Styling Metabox's custom fields using CSS

Hi,

i followed the tutorial here https://metabox.io/style-meta-boxs-custom-fields-using-css/

I'm using oxygen builder so lack a functions.php file to enqueue a custom css style sheet as described in your technique in the tutorial.

Adding custom css to oxygens style sheet does not work, neither did adding custom css to advanced scripts or scripts organiser. How do I style metabox's custom fields with CSS while using oxygen and not having a functions php file?