Generalize placeholder styles.

Give any direct child of demo-containre the same style, so it can be
used for both #placeholder and #overview.

examples/examples.css

@@ -75,7 +75,7 @@ input {
 	-webkit-box-shadow: 0 3px 10px rgba(0,0,0,0.1);
 }
 
-#placeholder {
+.demo-container > div {
 	width: 100%;
 	height: 100%;
 	font-size: 14px;