Hey Anna,
please paste these code according to your requirement:
Goto
wp-admin > appearance > customize > custom css > paste
1) if you want to change the color of background, use this code
div.pusher {background-color: #caff7f !important;}
refer:
http://prntscr.com/lhob23
** #caff7f is hex color code. change accordingly.
2) if you want to set background image then use this code
div.pusher {background-image: url('http://www.mysite.com/wp-content/uploads/main-banner.jpg') !important; background-attachment: fixed !important; background-size: cover !important; }
please copy the url from
wp-admin > media > library.
refer:
http://prntscr.com/lhoc1v
hope this helps.