Responsive github.com CSS. Use e.g Control Freak Chrome extension to inject this CSS.

responsive-github-com.css

.wrapper .header,
.site-footer {
  min-width: auto;
  padding: 10px;
}
body .container {
  max-width: 980px;
  width: auto;
}

.site {
  margin: auto;
  max-width: 1000px;
  padding-left: 10px;
  padding-right: 10px;
}
.site .container {
  width: auto;
}

.site .repository-with-sidebar.with-full-navigation .repository-content,
.site .repository-with-sidebar .repository-content {
  float: none;
  overflow: hidden;
  width: auto;
}
.site .repository-with-sidebar.with-full-navigation .repository-content {
	margin-right: 180px;
}
.site .repository-with-sidebar .repository-content {
  margin-right: 48px;
}

@media all and (max-width: 1014px) {
  td.age {
    display: none;
  }
  
  .subnav-search input.subnav-search-input {
    width: 142px;
  }
}

@media all and (max-width: 768px) {
  .site .repository-with-sidebar.with-full-navigation .repository-content,
	.site .repository-with-sidebar .repository-content {
    margin-right: 0;
  }
  .header .site-search {
  	float: none;
    margin-bottom: 10px;
    overflow: hidden;
  }
  .header .site-search form {
    width: 100%;
  }
  .site .repository-with-sidebar .repository-sidebar,
  .site .repository-with-sidebar.with-full-navigation .repository-sidebar{
  	float: none;
  	width: 100%;
  }
  .sunken-menu-group .sunken-menu-item {
  	margin-bottom: 0;
    margin-top: 0;
  	text-align: center;
  }
	.sunken-menu-group .sunken-menu-item.selected {
    border-color: transparent #eee #eee #eee;
    border-radius: 0 0 3px 3px;
    box-shadow: 1px 1px 3px rgba(0,0,0,0.05);
  }
	.sunken-menu-group .sunken-menu-item.selected:after {
  	height: 3px;
  	left: 0;
    top: auto;
    width: auto;
  }
	.sunken-menu .sunken-menu-item:focus,
	.sunken-menu .sunken-menu-item:hover {
    text-decoration: none;
  	box-shadow: inset 0px -2px 0 #ccc;
  }
	nav.sunken-menu {
  	background: transparent;
    box-shadow: none;
  	display: table;
    padding-top: 0;
  	width: 100%;
  }
  nav.sunken-menu:before {
    display: none;
  }
  .sunken-menu-group,
	.sunken-menu-separator {
  	display: table-row;
  }
	.sunken-menu-group > li {
  	display: table-cell;
  }
  .sunken-menu .sunken-menu-separator {
    background: none;
	}
  .sunken-menu .sunken-menu-separator:before {
    background-image: -webkit-linear-gradient(left, #fff 0%, #eee 50%, #fff 100%);
    background-image: linear-gradient(to right, #fff 0%, #eee 50%, #fff 100%);
  	top: auto;
  }
  .with-full-navigation .repo-nav .sunken-menu-group .full-word {
  	width: auto;
  }
  .site .pagehead ul.pagehead-actions {
    float: none;
    margin: 0 -5px 10px -5px;
    overflow: hidden;
    width: 100%;
  }
  .site .pagehead ul.pagehead-actions > li {
  	margin: 0;
  	padding: 0 5px;
  	position: relative;
  	width: 33.3%;
  }
	.pagehead-actions > li .btn-with-count {
    border-top-right-radius: 3px;
    border-bottom-right-radius: 3px;
  	display: block;
  	float: none;
    text-align: left;
    width: 100%;
  }
	.pagehead-actions > li .social-count {
  	border-left: 1px solid #ddd;
  	float: none;
    position: absolute;
    right: 0;
    top: 0;
  }

  .subnav>div.right {
  	float: none;
    margin-bottom: 10px;
  	margin-left: 0;
    overflow: hidden;
  }
  .subnav-search.left {
  	float: none;
    overflow: hidden;
  }
	.subnav-search input.subnav-search-input {
    width: 100%;
  }
  
  .subnav-links.left {
  	float: none;
    width: 100%;
  }
  .subnav-links.left a {
  	width: 25%;
  }
  
  .repo-container.repository-with-sidebar .repository-sidebar .sidebar-button {
  	float: left;
    width: 48%;
  }
  .repository-with-sidebar .repository-sidebar .sidebar-button ~ .repository-with-sidebar .repository-sidebar .sidebar-button {
    float: right;
  }
  body .site-footer {
  	padding-top: 80px;
  }
  .site-footer .site-footer-links {
  	float: none;
  	margin-bottom: 10px;
    text-align: center;
  }
}