Add scrollbar to the navigation bar (#3237)

_static/css/custom.css

@@ -21,6 +21,7 @@
     --navbar-level-2-color: #b8d6f0;
     --navbar-level-3-color: #a3c4e1;
     --navbar-heading-color: #ff7381;
+    --navbar-scrollbar-background: #131e2b;
 
     --link-color: #2980b9;
     --link-color-hover: #3091d1;
@@ -87,6 +88,7 @@
         --navbar-level-2-color: #ccc;
         --navbar-level-3-color: #bbb;
         --navbar-heading-color: #ee7381;
+        --navbar-scrollbar-background: #1c1e21;
 
         --link-color: #8cf;
         --link-color-hover: #9df;
@@ -649,3 +651,35 @@ code,
     font-weight: inherit;
     padding: 0;
 }
+
+/* allows the navbar's scrollbar to be shown */
+nav.wy-nav-side, .wy-side-scroll, .rst-versions {
+    overflow-x: unset;
+    overflow-y: unset;
+    width: 308px;
+}
+nav.wy-nav-side {
+    overflow-x: hidden;
+    min-height: calc(100% - 43px);
+    margin-bottom: 50rem;
+    padding-bottom: unset;
+}
+.wy-side-nav-search {
+    width: unset;
+}
+.rst-versions {
+    height: 43px;
+}
+/* navbar scrollbar styling */
+nav.wy-nav-side {
+    scrollbar-color: var(--navbar-heading-color) var(--navbar-scrollbar-background);
+}
+nav.wy-nav-side::-webkit-scrollbar {
+    width: .75rem;
+}
+nav.wy-nav-side::-webkit-scrollbar-track {
+    background-color: var(--navbar-scrollbar-background);
+}
+nav.wy-nav-side::-webkit-scrollbar-thumb {
+    background-color: var(--navbar-heading-color);
+}