Change some default playground styling

Reviewed By: danielbuechele

Differential Revision: D6977989

fbshipit-source-id: 64975685490b8a2230d4effc8791c028b680d2b7

website/src/components/Playground/index.css

@@ -15,6 +15,7 @@
 }
 
 .Playground {
+  display: flex;
   flex-grow: 1;
   position: relative;
   width: 100%;
@@ -41,3 +42,10 @@
   background-size: 10px 10px, 10px 10px, 100px 100px, 100px 100px, 100px 100px,
     100px 100px, 100px 100px, 100px 100px;
 }
+
+
+.Playground > .YogaNode {
+  margin: auto;
+  position: static;
+  align-self: center;
+}