NNTP: Fix the issue where the lower corners had a fex dark pixels...

chrome/browser/resources/new_tab_theme.css

 html {
   background-image:url(chrome://theme/theme_ntp_background?$1);
   background-color:$2;
   background-position:$3;
   background-repeat:$5;
   overflow:hidden;
 }
 html[bookmarkbarattached='true'] {
   background-position:$4;
 }
@@ skipping 18 matching lines @@
 .thumbnail-container:focus .edit-bar,
 .thumbnail-container:hover .edit-bar {
   background-color: $6;
   background-image: -webkit-gradient(linear, left top, left bottom,
                                      from($7),
                                      to($6));
 }
 .thumbnail-container:hover > .edit-mode-border {
   background-color: $6;
 }
-.edit-mode-border {
-  background-color: $7;
-}
 .section {
   border: 1px solid $7;
 }
 .section > h2 {
   background-color: $7;
   background-image: -webkit-gradient(linear, left top, left bottom,
                                      from($7),
                                      to($6));
   color: $$1;
 }
 #view-toolbar > input {
   background-color: $7;
   -webkit-mask-image: url(chrome://theme/newtab_menu_mask);
 }
 #option-menu > [selected] {
   background-color: $7;
   color: white;
 }
 .tips-title {
   color: $$1;
 }