[WebUI] Fix rest of CSS style nits so I can turn on CSS checker.

chrome/browser/resources/print_preview/margins.css

 /* Copyright (c) 2012 The Chromium Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 .margin-box {
   background-color: #2a2a2a;
   border: 1px solid #888;
   box-sizing: border-box;
   color: white;
@@ skipping 25 matching lines @@
   /* -height + 1 */
   margin-top: -23px;
 }
 
 .margins-ui-pair.right .margin-box {
   /* -width - 2 */
   margin-left: -58px;
 }
 
 .margin-box.invalid {
-  background-color: #c11b17;
+  background-color: rgb(193, 27, 23);
 }
 
 .margins-ui-pair {
   background-color: transparent;
   border-color: transparent;
   position: absolute;
 }
 
 .margins-ui-pair.right,
 .margins-ui-pair.left {
   cursor: ew-resize;
 }
 
 .margins-ui-pair.top,
 .margins-ui-pair.bottom {
   cursor: ns-resize;
 }
 
 .margins-ui-pair.dragging {
   z-index: 1;
 }
 
 .margin-line {
-  border-color: #4080FA;
+  border-color: rgb(64, 128, 250);
   border-style: dashed;
   border-width: 1px;
   pointer-events: none;
   position: absolute;
 }
 
 .margins-ui-pair.top .margin-line,
 .margins-ui-pair.bottom .margin-line {
   height: 0;
   left: 0;
   top: 50%;
   width: 100%;
 }
 
 .margins-ui-pair.left .margin-line,
 .margins-ui-pair.right .margin-line {
   height: 100%;
   left: 50%;
   top: 0;
   width: 0;
 }
 
 #customized-margins {
   position: absolute;
   top: 0;
 }
 
 #customized-margins.invisible {
+  opacity: 0;
   pointer-events: none;
-  opacity: 0;
 }