I think you mean these class definitions in style.css:
/* =Media queries
-------------------------------------------------------------- */
/* Minimum width of 600 pixels. */
@media screen and (min-width: 600px) {
...cut...
.site {
margin: 0 auto;
max-width: 960px;
max-width: 68.571428571rem;
overflow: hidden;
}
...cut...
}
/* Minimum width of 960 pixels. */
@media screen and (min-width: 960px) {
...cut...
body .site {
padding: 0 40px;
padding: 0 2.857142857rem;
margin-top: 48px;
margin-top: 3.428571429rem;
margin-bottom: 48px;
margin-bottom: 3.428571429rem;
box-shadow: 0 2px 6px rgba(100, 100, 100, 0.3);
}
...cut...
}
Edit:
Here is a style.css for a child theme:
/*
Theme Name: Twenty Twelve Child
Theme URI: http: //example.com/
Description: Child theme for the Twenty Twelve theme
Author: birgire
Template: twentytwelve
Version: 1.0
*/
@import url("../twentytwelve/style.css");
/*-----------------
Custom page layout
-------------------*/
/* remove site margin */
body .site{margin:0;max-width: 100%;}