#1. Mobile Only
Use this code to Custom CSS box
@media screen and (max-width:767px) { div.header-menu { left: unset; width: 70%; } }
Result:
#2. Desktop Only
In case you use code to force the Mobile menu on the Desktop, to reduce the Mobile menu width on the Desktop Only, you can use this CSS code
@media screen and (min-width: 768px) { div.header-menu { left: unset; width: 30%; } }