Improve the sub-menu arrows for the tree context menu

src/public/app/menus/context_menu.js

@@ -3,6 +3,7 @@ import keyboardActionService from '../services/keyboard_actions.js';
 class ContextMenu {
     constructor() {
         this.$widget = $("#context-menu-container");
+        this.$widget.addClass("dropend");
         this.dateContextMenuOpenedMs = 0;
 
         $(document).on('click', () => this.hide());

src/public/stylesheets/style.css

@@ -244,6 +244,11 @@ div.ui-tooltip {
     box-shadow: unset;
 }
 
+.dropend .dropdown-toggle::after {
+    margin-left: .5em;
+    color: var(--muted-text-color);
+}
+
 .dropdown-menu .disabled {
     color: #888 !important;
     pointer-events: none;