Fix the tree background color for the cities demo (#84)

* fix the tree background color for the cities demo, if browser uses prefers-color-scheme: dark
* set the focused node text to black

Author: rafma0

packages/showcase/styles/cities.module.css

@@ -213,5 +213,14 @@
     .mobileWarning {
         display: block;
     }
-  }
+}
 
+@media (prefers-color-scheme: dark) {
+    .tree {
+        background: #101010;
+    }
+
+    .row:focus-visible .node {
+        color: black;
+    }
+}