Set background color in index.html to match Slint

gui/index.html

@@ -8,6 +8,10 @@
             init();
         </script>
         <style>
+            body {
+                /* same background color as the Slint app */
+                background: #1b1b1b;
+            }
             #canvas {
                 /* Force Slint to occupy the entire viewport */
                 width: 100vw !important;