Add y-axis scroll function.

src/perfect-scrollbar.css

@@ -0,0 +1,15 @@
+.ps-container .ps-scrollbar-y {
+    position: absolute;
+    width: 10px;
+    right: 5px;
+    background-color: gray;
+}
+
+.ps-container .ps-scrollbar-y:hover {
+    background-color: black;
+    cursor: default;
+}
+
+.ps-container .ps-scrollbar-y.in-scrolling {
+    background-color: black;
+}