Hi there,
if you want to use CSS then use max-width: 1200px instead of width as that will overflow on smaller screens.
and its a perfectly valid method, but there are other methods.
Option 1 – use a Layout Element to set the Content -> Container Width to 1200px.
This is similar to your CSS in that it will reduce only the content width:
Option 2 – make the site header bigger with this CSS:
#masthead .inside-header {
max-width: 1400px;
For the Hero section, you cans set the inner Container max-width to 1400px.
I like Option 2 as you can leave the Customizer to control the main content widths.
And you can always combine it with Option 1 if there are use cases where the content on specific pages need to be less wide.