Use the same border color throughout the app

With this change, all CSS border declarations that specify some shade of
gray are modified to use the one unified --border-color declaration in
app-theme.html.

Further followup with UX is required to ensure that the shade of gray
chosen is the correct one -- for now, the most-used one is used.

Change-Id: I2134ebf4a459ba040e01856c11389780307d8648
diff --git a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
index 2aaaae5..9fd564c 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
@@ -58,7 +58,7 @@
       }
       table {
         border-collapse: collapse;
-        border-right: 1px solid #ddd;
+        border-right: 1px solid var(--border-color);
         table-layout: fixed;
       }
       .lineNum {