Introduce a border-radius css var and use it everywhere

Sets border-radius to 4px as the app-theme default. The only
border-radii that stay different from 4px are the circle shapes for
account and voting chips.

Change-Id: I3417b141aaa19440300f0628eca1a9229eb6c131
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 f63a679..7aac01c 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
@@ -222,7 +222,7 @@
       .content .trailing-whitespace,
       .trailing-whitespace .intraline,
       .content .trailing-whitespace .intraline {
-        border-radius: .4em;
+        border-radius: var(--border-radius, 4px);
         background-color: var(--diff-trailing-whitespace-indicator);
       }
       #diffHeader {