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/change/gr-message/gr-message.html b/polygerrit-ui/app/elements/change/gr-message/gr-message.html
index 9457ed0..acbee7f 100644
--- a/polygerrit-ui/app/elements/change/gr-message/gr-message.html
+++ b/polygerrit-ui/app/elements/change/gr-message/gr-message.html
@@ -150,7 +150,7 @@
       }
       .score {
         border: 1px solid rgba(0,0,0,.12);
-        border-radius: 3px;
+        border-radius: var(--border-radius);
         color: var(--primary-text-color);
         display: inline-block;
         margin: -1px 0;