[MD settings] change layout of Custom Font settings

chrome/browser/resources/settings/settings_shared_css.html

 <!-- Common styles for Material Design settings. -->
 <dom-module id="settings-shared">
   <template>
     <style>
       :root {
         --checkbox-margin-start: 2px;
         --checkbox-size: 16px;
         --checkbox-spacing: 18px;
         --settings-secondary: {
           color: var(--paper-grey-500);
@@ skipping 116 matching lines @@
         @apply(--settings-secondary);
       }
 
       .list-item {
         @apply(--layout-center);
         display: flex;
         min-height: 40px;
         padding: 0;
       }
 
+      .list-item.underbar {
+        border-bottom: 1px solid var(--paper-grey-300);
+      }
+
       .list-item iron-icon {
         @apply(--layout-center);
         height: 20px;
         width: 20px;
       }
 
       .list-item select {
         -webkit-margin-start: 4px;
       }
 
@@ skipping 92 matching lines @@
 
       .settings-checkbox-spacer {
         -webkit-margin-start: calc(
             var(--checkbox-margin-start) +
             var(--checkbox-size) +
             var(--checkbox-spacing));
       }
     </style>
   </template>
 </dom-module>