Move coverage colors to theme css vars

Change-Id: I80dde321bb923ddca32773350b8c8464043f96cd
diff --git a/polygerrit-ui/app/styles/themes/dark-theme.html b/polygerrit-ui/app/styles/themes/dark-theme.html
index ceb42a9..69a787e 100644
--- a/polygerrit-ui/app/styles/themes/dark-theme.html
+++ b/polygerrit-ui/app/styles/themes/dark-theme.html
@@ -105,6 +105,8 @@
       --light-rebased-add-highlight-color: #487165;
       --light-remove-add-highlight-color: #2f3f2f;
       --light-remove-highlight-color: #320404;
+      --coverage-covered: #e0f2f1;
+      --coverage-not-covered: #ffd1a4;
 
       /* syntax colors */
       --syntax-attr-color: #80cbbf;