add MessageBox doc

doc/themes/next/source/css/_custom/custom.styl

@@ -1,5 +1,9 @@
 // Custom styles.
 
+.container{
+  min-height: 100%;
+}
+
 .container:before{
   content: "";
   opacity: 0.1;
@@ -8,9 +12,10 @@
   bottom: 0;
   right: 0;
   position: absolute;
-  z-index: -1; 
+  z-index: -1;
   background: url(https://raw.githubusercontent.com/HandyOrg/HandyOrgResource/master/HandyControl/Resources/cloud.png);
 }
+
 .posts-expand .post-body img {
   margin: 3px;
   display: inline;