Add scrollbar to the content and sidebar

Author: Dedelweiss

scaladoc/resources/dotty_res/styles/theme/layout/container.css

@@ -20,6 +20,10 @@ p {
 }
 
 ::-webkit-scrollbar {
-  width: 0;
+  width: 5;
   background: transparent;
 }
+
+::-webkit-scrollbar-thumb {
+  background: black;
+}