[func] add scrollbar to TOC

static/css/main.css

@@ -219,6 +219,10 @@ body {
   background-color: var(--main-color) !important;
 }
 
+/*******************
+    books menu
+*******************/
+
 .dropdown-menu.dropdown-chapters {
   min-width: 15rem;
   max-height: 30rem;
@@ -266,6 +270,19 @@ body {
 
 }
 
+.dropdown-menu {
+  padding: 0;
+}
+
+.accordion {
+  overflow-y: scroll;
+  max-height: 30rem;
+}
+
+/*******************
+    chapters menu
+*******************/
+
 .accordion-button {
   padding: 0.5rem 1rem;
   color: var(--main-color);