I have been trying to figure this out. I made some progress, but did not find a solution yet. Just sharing where I am so far.
The code below DOES set the page width when I adjust the max-width to a percentage, but it includes all pages (home, posts, etc.).
I want to leave the home page at full width, and be sure any changes I make do not affect mobile devices.
[stylesheet code lines 645-650]
.site-inner,
.wrap {
float: none;
margin: 0 auto;
max-width: 1200px;
}