Make GR-DIFF-COMMENT-THREAD more visually distinct

Bug: Issue 4793
Change-Id: I102b6e3ccd3ec71bb28cbc3148e02a6a389256b0

polygerrit-ui/app/elements/diff/gr-diff-comment-thread/gr-diff-comment-thread.html

diff --git a/polygerrit-ui/app/elements/diff/gr-diff-comment-thread/gr-diff-comment-thread.html b/polygerrit-ui/app/elements/diff/gr-diff-comment-thread/gr-diff-comment-thread.html
index 25237b5..d0a49dd 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-comment-thread/gr-diff-comment-thread.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff-comment-thread/gr-diff-comment-thread.html
@@ -22,8 +22,7 @@
   <template>
     <style>
       :host {
-        border: 1px solid #ddd;
-        border-right: none;
+        border: 1px solid #bbb;
         display: block;
         white-space: normal;
       }