change default color of controls to match legacy

css/reveal.css

@@ -239,7 +239,7 @@ body {
   right: 12px;
   left: auto;
   z-index: 1;
-  color: #fff;
+  color: #000;
   pointer-events: none;
   font-size: 10px; }
   .reveal .controls button {

css/reveal.scss

@@ -275,7 +275,7 @@ $controlsArrowAngleActive: 36deg;
 	right: $spacing;
 	left: auto;
 	z-index: 1;
-	color: #fff;
+	color: #000;
 	pointer-events: none;
 	font-size: 10px;