Add CSS style for permanent link anchors

static/css/style.css

@@ -378,3 +378,19 @@ img {
      border-bottom: medium none;
      color: rgb(0, 85, 128);
 }
+
+a.headerlink {
+    visibility:hidden;
+    padding-left:0.5em;
+    color:grey;
+}
+
+h1:hover > a.headerlink,
+h2:hover > a.headerlink,
+h3:hover > a.headerlink,
+h4:hover > a.headerlink,
+h5:hover > a.headerlink,
+h6:hover > a.headerlink,
+dt:hover > a.headerlink {
+    visibility: visible;
+}