LESS:
-----
@width: 960px;
@media all and (max-width: 768px) {
  @width: 760px;
}
body {
  width: @width;
}


CSS:
----
body {
  width: 960px;
}
@media all and (max-width: 768px) {
  body {
    width: 760px;
  }
}