Fix banner color in dark mode

Before the change, the background color for the banner was not changing
when switching to dark mode. Use appropriate variable from core, so that
the color changes depending on if the user has selected a Light or Dark
theme.

Change-Id: I4892f23949bab3c2609108e89be50e8be47cb7e5

gr-messageoftheday/gr-messageoftheday-banner_html.js

diff --git a/gr-messageoftheday/gr-messageoftheday-banner_html.js b/gr-messageoftheday/gr-messageoftheday-banner_html.js
index 5174877..c0230bd 100644
--- a/gr-messageoftheday/gr-messageoftheday-banner_html.js
+++ b/gr-messageoftheday/gr-messageoftheday-banner_html.js
@@ -18,7 +18,7 @@
 export const htmlTemplate = Polymer.html`
 <style include="shared-styles">
   #container {
-    background-color: lightyellow;
+    background-color: var(--assignee-highlight-color);
     display: flex;
     height: fit-content;
     justify-content: space-between;