only offset slides vertically when center option is true

css/reveal.css

@@ -560,7 +560,7 @@ body {
 	position: absolute;
 	width: 100%;
 	height: 100%;
-	top: -40px;
+	top: 0;
 	right: 0;
 	bottom: 0;
 	left: 0;
@@ -624,6 +624,10 @@ body {
 	display: block;
 }
 
+.reveal.center .slides {
+	top: -40px;
+}
+
 .reveal.center .slides,
 .reveal.center .slides section {
 	height: 100%;