Use a border with transparency on vote chips

Similar to the gr-buttons that represent votes, using a border with
transparency allows the border color to be derived from the button
color.

Change-Id: I9bb2bb45d403cd9eeb42d24204a2be9c839cec80

polygerrit-ui/app/elements/change/gr-message/gr-message.html

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 abefaf5..a73faa4 100644
--- a/polygerrit-ui/app/elements/change/gr-message/gr-message.html
+++ b/polygerrit-ui/app/elements/change/gr-message/gr-message.html
@@ -136,7 +136,7 @@
         padding: .5em 0 0 0;
       }
       .score {
-        border: 1px solid var(--border-color);
+        border: 1px solid rgba(0,0,0,.12);
         border-radius: 3px;
         color: var(--primary-text-color);
         display: inline-block;