could you please send us your website link along with your wordpress username and passsword ? we will need to do check the issue.
also could you please tell which background are you trying to change? is it a section or the page background itself ?
theme loads an index file which has all the dynamic css which is nothing but a variable named as $custom_css placed in a style.php file. but before you do any changes in the /inc/style.php file please share your wordpress credentials and let us check what is causing the issue.