You can hook client-side events that fire for various treeview events, such as selection, node checking, etc. This is achieved through the collection of the JQTreeView class. Each event is a string pointing to a javascript function that will execute when the event is fired. Each function typically receives two parameters - the node causing the event and the event argument of the browser (client-side event). Some events like Check, Select, etc can be cancelled by returning false from the event handler. Please, refer to the ASPX tab for additional details.
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="default.aspx.cs" Inherits="JQSuiteASPNETExample.examples.tree.clientevents._default" %>
<%@ Register Assembly="Trirand.Web" TagPrefix="trirand" Namespace="Trirand.Web.UI.WebControls" %>

<!DOCTYPE html>
<html lang="en">
<head id="Head1" runat="server">
    <meta charset="utf-8">
    <title>jqTreeView for ASP.NET WebForms - Text Only</title>
    <!-- The jQuery UI theme that will be used by the treeview  -->
    <link rel="stylesheet" type="text/css" media="screen" href="http://code.jquery.com/ui/1.12.1/themes/redmond/jquery-ui.css" />
    <!-- The jQuery UI theme extension jqTreeView needs -->
    <link rel="stylesheet" type="text/css" media="screen" href="/themes/ui.jqtreeview.css" />
    <!-- jQuery runtime minified -->
    <script src="/js/jquery-3.2.1.min.js" type="text/javascript"></script>    
    <!-- The jqTreeView client-side javascript -->
    <script src="/js/trirand/jquery.jqTreeView.min.js" type="text/javascript"></script>    
    <style type="text/css">    
        body, html { font-size: 80%; }    
    </style>
</head>

<body>
    <form id="form1" runat="server">
    
    <table>
        <tr>
            <td valign="top">
               <trirand:JQTreeView 
                    runat="server" 
                    ID="JQTreeView1" 
                    CheckBoxes="true"
                    Width="200"
                    Height="300">
                    <ClientSideEvents
                         Check="handleCheck"
                         Collapse="handleCollapse"
                         Expand="handleExpand"
                         MouseOut="handleMouseOut"
                         MouseOver="handleMouseOver"
                         Select="handleSelect"
                    />
                    <Nodes>
                        <trirand:JQTreeNode Text="One" />
                        <trirand:JQTreeNode Text="Two" Expanded="true">
                            <Nodes>
                                <trirand:JQTreeNode Text="2.1" />
                                <trirand:JQTreeNode Text="2.2" Expanded="true">
                                    <Nodes>
                                        <trirand:JQTreeNode Text="2.2.1" Value="London" />
                                        <trirand:JQTreeNode Text="2.2.2" />
                                        <trirand:JQTreeNode Text="2.2.3" />
                                    </Nodes>
                                </trirand:JQTreeNode>
                                <trirand:JQTreeNode Text="2.3" />
                            </Nodes>
                        </trirand:JQTreeNode>
                        <trirand:JQTreeNode Text="Three" />
                    </Nodes>
                </trirand:JQTreeView>             
            </td>
            <td valign="top" style="width:300px; height: 300px;">                
                <div id="eventLog" style="height:300px; overflow:auto; font-size:110%; font-family:Tahoma" />
            </td>
        </tr>
    </table>
    
    <script type="text/javascript">
    
    function handleCheck(node, event) {
        var tree = $("#JQTreeView1").getTreeViewInstance();
        var nodeOptions = tree.getNodeOptions(node);           
        if (nodeOptions.text == "Two") {
            writeLine(nodeOptions.text, " : <b>cannot be checked</b>.");
            return false;
        }
        
        writeLine(nodeOptions.text, " : <b>Check</b> fired. Checked state is : <b>" + nodeOptions.checked + "</b>");
    }
    
    function handleCollapse(node, event) {
        var tree = $("#JQTreeView1").getTreeViewInstance();
        var nodeOptions = tree.getNodeOptions(node);           
        if (nodeOptions.text == "2.2") {
            writeLine(nodeOptions.text, " : <b>cannot be collapsed</b>");
            return false;
        }
        
        writeLine(nodeOptions.text, " : <b>Collapse</b> fired");
    }
    
    function handleExpand(node, event) {
        var tree = $("#JQTreeView1").getTreeViewInstance();
        var nodeOptions = tree.getNodeOptions(node);           
        if (nodeOptions.text == "2.2") {
            writeLine(nodeOptions.text, " : <b>cannot be expanded</b>");
            return false;
        }
        
        writeLine(nodeOptions.text, " : <b>Expand</b> fired");
    }
    
    function handleMouseOut(node, event) {
        var tree = $("#JQTreeView1").getTreeViewInstance();
        var nodeOptions = tree.getNodeOptions(node);
        
        writeLine(nodeOptions.text, " : <b>MouseOut</b> fired");
    }
    
    function handleMouseOver(node, event) {
        var tree = $("#JQTreeView1").getTreeViewInstance();
        var nodeOptions = tree.getNodeOptions(node);           
        
        writeLine(nodeOptions.text, " : <b>MouseOver</b> fired");
    }
    
    function handleSelect(node, event) {
        var tree = $("#JQTreeView1").getTreeViewInstance();
        var nodeOptions = tree.getNodeOptions(node);           
        if (nodeOptions.text == "Two") {
            writeLine(nodeOptions.text," : <b>cannot be selected</b>");
            return false;
        }
        
        writeLine(nodeOptions.text, " : <b>Select</b> fired");
    }
    
    function writeLine(nodeText, message) {
        var currentText = $("#eventLog").html();
        $("#eventLog").html(message + " for node <b>" + nodeText + "</b><br>" + currentText);
    }
    
    </script>
    
    <br />
    <br />
    <trirand:codetabs runat="server" ID="tabs"></trirand:codetabs>
    </form>
</body>
</html>
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace JQSuiteASPNETExample.examples.tree.clientevents
{
    public partial class _default : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {

        }
    }
}
Switch theme:


Theming is based on the very popular jQuery ThemeRoller standard. This is the same theming mechanism used by jQuery UI and is now a de-facto standard for jQuery based components. The benefits of using ThemeRoller are huge. We can offer a big set of ready to use themes created by professional designers, including Windows-like themes (Redmond), Apple-like theme (Cupertino), etc. In addition to that any jQuery UI controls on the same page will pick the same theme.

Last, but not least, you can always roll your own ThemeRoller theme, using the superb Theme Editor

To use a theme, simply reference 2 Css files in your Html <head> section - the ThemeRoller theme you wish to use, and the jqGrid own ThemeRoller Css file. For example (Redmond theme):

<link rel="stylesheet" type="text/css" media="screen" href="/themes/redmond/jquery-ui-1.8.2.custom.css" />
<link rel="stylesheet" type="text/css" media="screen" href="/themes/ui.jqgrid.css" />