Add theme var --iron-overlay-backdrop-background-color

This ensures that dialogs get an overlay-backdrop that is actually
visible.

Change-Id: Ia89138e927a2b6e0d8188a6c967491bdee1b68da
diff --git a/polygerrit-ui/app/styles/themes/app-theme.html b/polygerrit-ui/app/styles/themes/app-theme.html
index 0ccb5c5..76ddac4 100644
--- a/polygerrit-ui/app/styles/themes/app-theme.html
+++ b/polygerrit-ui/app/styles/themes/app-theme.html
@@ -163,9 +163,13 @@
   --syntax-title-color: #0000c0;
   --syntax-type-color: #2a66d9;
   --syntax-variable-color: var(--primary-text-color);
+
   /* misc */
   --border-radius: 4px;
   --reply-overlay-z-index: 1000;
+
+  /* paper and iron component overrides */
+  --iron-overlay-backdrop-background-color: black;
   --iron-overlay-backdrop-opacity: 0.32;
   --iron-overlay-backdrop: {
     transition: none;
diff --git a/polygerrit-ui/app/styles/themes/dark-theme.html b/polygerrit-ui/app/styles/themes/dark-theme.html
index 513f28a..015d372 100644
--- a/polygerrit-ui/app/styles/themes/dark-theme.html
+++ b/polygerrit-ui/app/styles/themes/dark-theme.html
@@ -136,6 +136,9 @@
 
       /* misc */
 
+      /* paper and iron component overrides */
+      --iron-overlay-backdrop-background-color: white;
+
       /* rules applied to <html> */
       background-color: var(--view-background-color);
     }