Examples: Change background color in offscreen canvas demo

examples/webgl_worker_offscreencanvas.html

@@ -6,7 +6,7 @@
 		<meta name="viewport" content="width=device-width, user-scalable=no, minimum-scale=1.0, maximum-scale=1.0">
 		<style>
 			body {
-				background:#777;
+				background:#000000;
 				padding:0;
 				margin:0;
 				font-weight: bold;