You can se the LoadOnDemand property of each JQTreeNode to true - this will show an expand image before the node and clicking on it will issue a callback to the server in order to fetch the child nodes. This is commonly referred to as Ajax or LoadOnDemand loading of nodes and allows loading infinite amount of nodes step-by-step. When a node marked as LoadOnDemand is expanded, the NodeRequested server-side event is fired and you can use the event-argument to get the Value of the expanded (parent) node. Using the Value you can then load child nodes in the eventArgs.ChildNodes collection - the nodes will be sent to the client-side via Ajax.
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="default.aspx.cs" Inherits="JQSuiteASPNETExample.examples.tree.loadondemand._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 - load nodes on demand via Ajax</title>
    <!-- The jQuery UI theme that will be used by the treeview  -->
    <link rel="stylesheet" type="text/css" media="screen" href="http://ajax.aspnetcdn.com/ajax/jquery.ui/1.10.3/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="http://ajax.microsoft.com/ajax/jquery/jquery-2.0.3.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">
    <div>
        <trirand:JQTreeView
            runat="server"
            ID="JQTreeView1"
            Width="300"
            Height="350" onnodesrequested="JQTreeView1_NodesRequested">
        </trirand:JQTreeView>
        <br />
        <br />
        <trirand:codetabs runat="server" ID="tabs"></trirand:codetabs>
    </div>
    </form>
</body>
</html>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using Trirand.Web.UI.WebControls;

namespace JQSuiteASPNETExample.examples.tree.loadondemand
{
    public partial class _default : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            if (!Page.IsPostBack)
            {
                string startingPath = Server.MapPath("~/examples");

                var dirs = GetDirectories(startingPath);
                var files = GetFiles(startingPath);

                foreach (JQTreeNode node in dirs)
                    JQTreeView1.Nodes.Add(node);
                foreach (JQTreeNode node in files)
                    JQTreeView1.Nodes.Add(node);                
            }
        }

        protected void JQTreeView1_NodesRequested(object sender, JQTreeViewNodesRequestedEventArgs e)
        {
            string startingPath = e.ParentNodeValue;

            var dirs = GetDirectories(startingPath);
            var files = GetFiles(startingPath);

            foreach (JQTreeNode node in dirs)
                e.ParentNodes.Add(node);
            foreach (JQTreeNode node in files)
                e.ParentNodes.Add(node);
        } 

        private List GetDirectories(string startingPath)
        {
            System.IO.DirectoryInfo dir = new System.IO.DirectoryInfo(startingPath);

            IEnumerable dirList = dir.GetDirectories("*.*",
                    System.IO.SearchOption.TopDirectoryOnly);

            var dirs = from directory in dirList
                       orderby directory.Name
                       select new JQTreeNode
                       {
                           Text = directory.Name,
                           Value = directory.FullName,
                           LoadOnDemand = true,
                           ImageUrl = "/images/folder.png",
                           ExpandedImageUrl = "/images/folder-open.png"
                       };

            List nodes = new List();
            foreach (JQTreeNode node in dirs)
            {
                nodes.Add(node);
            }

            return nodes;
        }

        private List GetFiles(string startingPath)
        {
            System.IO.DirectoryInfo dir = new System.IO.DirectoryInfo(startingPath);

            IEnumerable fileList = dir.GetFiles("*.*",
                    System.IO.SearchOption.TopDirectoryOnly);

            var dirs = from file in fileList
                       orderby file.Name
                       select new JQTreeNode
                       {
                           Text = file.Name,
                           Value = file.FullName,
                           ImageUrl = "/images/document.png",
                       };

            List nodes = new List();
            foreach (JQTreeNode node in dirs)
            {
                nodes.Add(node);
            }

            return nodes;
        }            
    }
}
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" />