The Selected property of each JQListItem instance can be set to mark the item as selected. Upon posting back to the server, the SelectedIndexChanged event will fire. You can also check which item is selected using the SelectedIndex property.
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="default.aspx.cs" Inherits="JQSuiteASPNETExample.examples.dropdownlist.selection._default" %>
<%@ Register Assembly="Trirand.Web" TagPrefix="trirand" Namespace="Trirand.Web.UI.WebControls" %>

<!DOCTYPE html>
<html lang="en">
    <meta charset="utf-8">
    <title>jqDropDownList for ASP.NET WebForms - Selection</title>
    <!-- The jQuery UI theme that will be used by the dropdownlist  -->
    <link rel="stylesheet" type="text/css" href="" />    
    <!-- The jQuery UI theme extension dropdownlist needs -->
    <link rel="stylesheet" type="text/css" href="/themes/ui.jqdropdownlist.css" />
    <!-- jQuery runtime minified -->
    <script type="text/javascript" src="/js/jquery-3.2.1.min.js"></script>
    <!-- The jqDropDownList client-side javascript -->
    <script type="text/javascript" src="/js/trirand/jquery.jqDropDownList.min.js"></script>
    <style type="text/css">    
        body, html { font-size: 80%; }
    <form id="Form1" runat="server">
    <br /><br /><br />
    <span style="font-size:10pt; font-family:Verdana">
        <asp:Label runat="server" ID="Label1" />
    <br /><br />
    <asp:Button runat="server" ID="Button1" Text="Show Selected Item" 
        onclick="Button1_Click" />
    <br /><br /><br />
    <br /><br /><br />    
    <trirand:codetabs runat="server" ID="tabs"></trirand:codetabs>    
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.dropdownlist.selection
    public partial class _default : System.Web.UI.Page
        protected void Page_Load(object sender, EventArgs e)


        protected void Button1_Click(object sender, EventArgs e)
            JQListItem item = JQDropDownList1.Items[JQDropDownList1.SelectedIndex];
            Label1.Text = "You have selected " + item.Text + "";

        protected void JQDropDownList1_SelectedIndexChanged(object sender, Trirand.Web.UI.WebControls.SelectedIndexChangedEventArgs 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" />