Not good at adapting body width on different devices

Mobile version does NOT adapt to the device screen width. There is no way to implement a per device body width (e.g. desktop, phone). I tried several universal themes and they all have the same problem. Classic themes like “Penscratch 2” have a superior way to implement the body width for desktop and for phone. I guess that Automattic should add an option in the theme editor to select the correct body width based on the device where the theme will be displayed.