Add a border to hovercards

Otherwise it lacks definition in dark mode.

Screenshots:
Light: https://imgur.com/a/2PwNalG
Dark: https://imgur.com/a/Pg2ceTm

Change-Id: Ic6da4520f84b701e5b60f5bfb05fdff69e2b8d23
diff --git a/polygerrit-ui/app/elements/shared/gr-hovercard/gr-hovercard-shared-style.js b/polygerrit-ui/app/elements/shared/gr-hovercard/gr-hovercard-shared-style.js
index bb81cfd..a392691 100644
--- a/polygerrit-ui/app/elements/shared/gr-hovercard/gr-hovercard-shared-style.js
+++ b/polygerrit-ui/app/elements/shared/gr-hovercard/gr-hovercard-shared-style.js
@@ -36,6 +36,7 @@
          to pick up this consistent styling. */
       #container {
         background: var(--dialog-background-color);
+        border: 1px solid var(--border-color);
         border-radius: var(--border-radius);
         box-shadow: var(--elevation-level-5);
       }