adds dark mode

layouts/partials/wompum-demo.html

@@ -6,7 +6,7 @@
   <div class="mb-8">
     <label class="block mb-2">Enter text to generate a grid:</label>
     <input type="text" 
-           class="w-full p-2 border rounded font-iosevka bg-sand-100"
+           class="w-full p-2 border rounded font-iosevka bg-sand-100 dark:bg-gray-800"
            placeholder="Type something..."
            value="{{ .Site.Title}}"
            oninput="updateWompumDemoGrid(this.value)">