Add small margin below presentation transition buttons

computer-science-blog/css/main.css

@@ -14,6 +14,7 @@
   	display: -ms-flexbox;
   	display: -webkit-flex;
   	display: flex;
+  	margin-bottom: 15px;
 
   	-webkit-flex-flow: row wrap;
   	flex-flow: row wrap;