Change background-color of titles above code examples.

This should make it clearer that the title belongs to the code and not
the preceding text.

assets/blog.css

@@ -336,3 +336,9 @@ kbd
 {
     outline: 0.01em dotted var(--fg-dim);
 }
+
+/* Includes title above code examples */
+.listingblock
+{
+    background-color: var(--bg-dark);
+}