sketch.js: initial support for dark/light mode logic

sketch.js

@@ -42,7 +42,7 @@ class RotatingOnion {
     scale(this.scale);
     model(onion_stroke);
     push()
-      emissiveMaterial(0,0,0)
+      emissiveMaterial(backgroundColor)
       model(onion_fill);
     pop()
 
@@ -191,6 +191,9 @@ function setup() {
   textAlign(CENTER, CENTER);
   randomSeed()
 
+  // Background color
+  backgroundColor = [0, 0, 0];
+
   // Initialize camera to get its position
   camera = createCamera();
 
@@ -216,7 +219,7 @@ function setup() {
 }
 
 function draw() {
-  background(0);
+  background(backgroundColor);
   orbitControl();
   ambientLight(100);
   shininess(0);