Remove #page CSS from Seven

We currently have this CSS in the Seven style.css:

/**
 * Page layout.
 */
#page {
  background: #fff;
  color: #333;
  margin-left: 0.8125em;
  margin-right: 0.8125em;
  position: relative;
}
@media screen and (min-width:28.125em) { /* 450px */
  #page {
    margin-left: 1.25em;
    margin-right: 1.25em;
  }
}
@media screen and (min-width:45em) { /* 720px */
  #page {
    margin-left: 2.5em;
    margin-right: 2.5em;
  }
}

We don't need any of it, the margins are now being handled by the generic class .layout-container in layout.css.

background: #fff;
color: #333;

The default browser background is white, so this is not needed. The color declaration should go on the the body tag in seven.base.css.

Remaining tasks

Remove all #page CSS from Seven
Add color: #333 to the body tag in seven.base.css
Remove #page from Seven's page.html.twig file