Make blank portions of diff themable

This makes it possible to call them out specifically.
For now, the colors in our two themes remain unchanged.

Change-Id: Icfc6b602be766d56b7a768c0a221b4f3b8fd6c43
diff --git a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
index a6e703b..4ec460f 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
@@ -80,10 +80,12 @@
         background-color: var(--diff-selection-background-color);
         color: var(--primary-text-color);
       }
-      .blank,
       .content {
         background-color: var(--view-background-color);
       }
+      .blank {
+        background-color: var(--diff-blank-background-color);
+      }
       .image-diff .content {
         background-color: var(--table-header-background-color);
       }