MM-52544 Fix missing reply bar mention highlight (#23534)

webapp/channels/src/sass/components/_post.scss

@@ -1122,11 +1122,12 @@
     &.post--comment {
         .post__body {
             padding-left: 7px;
-            border-left: 4px solid $gray;
+            border-left: 4px solid rgba(var(--center-channel-color-rgb), 0.2);
+        }
 
-            &.mention-comment {
-                border-color: $yellow;
-                border-left: 4px solid $yellow;
+        &.mention-comment {
+            .post__body {
+                border-left: 4px solid var(--mention-highlight-bg);
             }
         }