themes/classic/sass/default/partials/_page.scss
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 html {
   background: $page-bg inline-image('line-tile.png') top left;
 }
 body {
   border: 0 0 1px 0 solid darken($page-bg, 5);
   > div {
     background-color: $sidebar-bg;
     border-bottom: 1px solid $nav-border-bottom;
     > div {
       background-color: $main-bg;
       border-right: 1px solid $sidebar-border;
     }
   }
 }
 
 @media only screen and (min-width: 1400px) {
   body { border: 0 1px 0 solid darken($page-bg, 5); }
 }