Actually i don’t see that warning message on my test site even setting WP debug to true but i saw the theme file that you are referring and could confirm logical error therefore notified the theme developer about this so that it will be fixed.
The issue will be fixed ASAP.
Thank you for notifying the issue. Your help here is really appreciated.
In the meanwhile to fix the issue edit the theme file /inc/templates/render_css.php on line number 44 and remove the code + 30 from it.