Use view-background-color throughout the app

Change-Id: Ib74162411afde7cdb2121f5e7b78e81a97341bfd
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.html b/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.html
index 839d3f6..e20a765 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.html
@@ -57,7 +57,7 @@
         }
       }
       gr-fixed-panel {
-        background-color: #fff;
+        background-color: var(--view-background-color);
         border-bottom: 1px solid var(--border-color);
         z-index: 1;
       }
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 008a85b..f45b2d3 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
@@ -87,7 +87,7 @@
       }
       .blank,
       .content {
-        background-color: #fff;
+        background-color: var(--view-background-color);
       }
       .full-width {
         width: 100%;