Change relative width of site-main section and sidebar section

Hi, I notice that the site-main section has a width set of:

.has-right-sidebar .site-main, .has-left-sidebar .site-main {
width: 64.5833333333%;
}

and the sidebar has a width set of:

sidebar #sidebar-right, .has-left-sidebar #sidebar-left, .has-left-sidebar #sidebar-right {
width: 25.8333333333%;
}

I would prefer to have the site-main section wider (eg 75%), and the sidebar more narrow (eg 20%).

I am using a child theme and I notice that if I change these percentages, it breaks the theme – the layout does not adapt properly when I narrow the screen to a mobile width.

Is there a particular way to change these values and not break the theme?

Thanks ahead of time 🙂