add RelativePanel, Rate, RunningBlock and ScrollViewer docs

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

@@ -1,5 +1,23 @@
 // Custom styles.
 
+::-webkit-scrollbar {
+  width: 8px;
+  height: 8px;
+}
+
+::-webkit-scrollbar-thumb {
+  border-radius: 4px;
+  background: #bdbdbd;
+}
+
+::-webkit-scrollbar-thumb:hover {
+  background: #757575;
+}
+
+::-webkit-scrollbar-thumb:active {
+  background: #212121;
+}
+
 .header-inner {
   box-shadow: 0 4px 8px 1.5px rgba(0,0,0,0.15);
   border-radius: 0 0 10px 10px;