Add helpful admonition CSS.

docs/_static/style.css

@@ -278,6 +278,10 @@ div.danger, div.error, div.warning {
   border: 1px solid #f66;
 }
 
+div.admonition {
+  padding: 10px;
+}
+
 p.admonition-title {
   display: inline;
 }
@@ -286,6 +290,20 @@ p.admonition-title:after {
   content: ':';
 }
 
+/* helpful admonitions */
+div.helpful {
+  background-color: #e4f2ff;
+  border: 1px solid #66b3ff;
+}
+
+div.helpful > p.admonition-title {
+  display: block;
+}
+
+div.helpful > p.admonition-title:after {
+  content: unset;
+}
+
 pre {
   background-color: #f5f5f5;
   border: 1px solid #C6C9CB;