Source: https://divibooster.com/changing-the-website-content-width-on-mobiles/

Divi doesn’t have a setting to change the mobile content width, it’s fixed at 80%. To change it:

@media only screen and (max-width: 980px) {
	#top-header > .container,
	#main-header > .container,
	#et_search_outer > .container,
	#main-content > .container,
	#main-footer > .container,
	#et-footer-nav > .container,
	#footer-bottom > .container,
	div.et_pb_row,
	div.et_pb_row.et_pb_row_fullwidth {
	width: 90% !important; 
   }
}
Code language: CSS (css)