DOMUI Prefs: Add a visual indication of the lead element

chrome/browser/resources/options/options_page.css

 body {
   -webkit-user-select: none;
   cursor: default;
   font-size: 100%;
 }
 
 #settings-title {
   -webkit-padding-end: 24px;
   color: #53637d;
   font-size: 200%;
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 }
 
 list > [selected],
 list:focus > [selected],
 list:focus > [lead][selected],
 list > [selected]:hover {
   background-color: #f2f2f2;
   background-image: none;
 }
 
+list:focus > [lead],
+list:focus > [lead][selected] {
+  border-top: 1px solid #b2b2b2;
+  border-bottom: 1px solid #b2b2b2;
+}
+
+list:focus > [lead]:nth-child(2),
+list:focus > [lead][selected]:nth-child(2) {
+  border-top: 1px solid #f2f2f2;
+}
+
+list:focus > [lead]:nth-last-child(2),
+list:focus > [lead][selected]:nth-last-child(2) {
+  border-bottom: 1px solid #f2f2f2;
+}
+
 list[disabled] > [lead][selected],
 list[disabled]:focus > [lead][selected] {
   border: none;
 }
 
 list[disabled] {
   opacity: 0.6;
 }
 
 list > .heading {
@@ skipping 113 matching lines @@
 }
 
 .suboption {
   -webkit-margin-start: 16px;
 }
 
 html[os=mac] label > input[type=checkbox],
 html[os=mac] label > input[type=radio] {
   margin-top: 2px;
 }