Set chip background color explicitly

Prevents strange coloration resulting from a theme styling the header.

Bug: Issue 8955
Change-Id: I29c9d17a169128ecc11a264927798373d3e2f3ed
diff --git a/polygerrit-ui/app/styles/themes/app-theme.html b/polygerrit-ui/app/styles/themes/app-theme.html
index 69262c9..4500e10 100644
--- a/polygerrit-ui/app/styles/themes/app-theme.html
+++ b/polygerrit-ui/app/styles/themes/app-theme.html
@@ -42,7 +42,7 @@
   --table-header-background-color: #fafafa;
   --table-subheader-background-color: #eaeaea;
 
-  --chip-background-color: var(--header-background-color);
+  --chip-background-color: #eee;
 
   --dropdown-background-color: #fff;