Adding items to a dropdownlist instance is easy - just create a new JQListItem instance for each item, set its Text property and add it to the Items collection of the dropdownlist instance.
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="default.aspx.cs" Inherits="JQSuiteASPNETExample.examples.multiselect.templates._default" %>
<%@ Register Assembly="Trirand.Web" TagPrefix="trirand" Namespace="Trirand.Web.UI.WebControls" %>

<!DOCTYPE html>
<html lang="en">
<meta charset="utf-8">
    <title>jqMultiSelect for ASP.NET WebForms - Templates</title>
    <!-- The jQuery UI theme that will be used by the combobox  -->
    <link rel="stylesheet" type="text/css" href="" />    
    <!-- The jQuery UI theme extension the multiselect needs -->
    <link rel="stylesheet" type="text/css" href="/themes/ui.jqmultiselect.css" />
    <!-- jQuery runtime minified -->
    <script type="text/javascript" src="/js/jquery-3.2.1.min.js"></script>
    <!-- The jQuery template engine -->    
    <script type="text/javascript" src=""></script>
    <!-- The jqMultiSelect client-side javascript -->
    <script type="text/javascript" src="/js/trirand/jquery.jqmultiselect.min.js"></script>
    <style type="text/css">    
        body, html { font-size: 80%; }
    <form id="Form1" runat="server">

     <script id="movieTemplate" type="text/x-jquery-tmpl"> 
        <table cellpadding="0" celspacing="0" style="width:350px ">
                <td rowspan="2" valign="middle" style="width:50px"><img src="${image}" /></td>
                <td style="width:300px"><b>${text}</b> (${year})</b><br>${actors}</td>
    <script id="alternativeMovieTemplate" type="text/x-jquery-tmpl"> 
        <table cellpadding="0" celspacing="0" style="width:320px ">
                <td style="width:250px"><b>${text}</b> (${year})</b><br>${actors}</td>
                <td rowspan="2" align="right" valign="middle" style="width:50px"><img src="${image}" /></td>
    <script id="headerTemplate" type="text/x-jquery-tmpl"> 
        <div class="ui-widger ui-state-default ui-corner-all" align="center"> Select an item </div>

    <script id="footerTemplate" type="text/x-jquery-tmpl"> 
        <div class="ui-widger ui-state-default ui-corner-all" style="padding:5px">Please select a movie</div>

    <table cellpadding="10" cellspacing="10">
            <td valign="top">
                        <trirand:JQListItem Text="American Beauty" ItemTemplateID="movieTemplate">
                                <trirand:JQListItemOption Name="year" Value="1999" />
                                <trirand:JQListItemOption Name="actors" Value="Kevin Spacey, Annette Benning" />
                                <trirand:JQListItemOption Name="image" Value="american_beauty.jpg" />
                            Text="Lost in Translation"
                                <trirand:JQListItemOption Name="year" Value="2003" />
                                <trirand:JQListItemOption Name="actors" Value="Bill Murray, Scarlett Johansson" />
                                <trirand:JQListItemOption Name="image" Value="lost_in_translation.jpg" />
                        <trirand:JQListItem Text="Fight Club" ItemTemplateID="movieTemplate">               
                                <trirand:JQListItemOption Name="year" Value="1999" />
                                <trirand:JQListItemOption Name="actors" Value="Brad Pitt, Edward Norton" />
                                <trirand:JQListItemOption Name="image" Value="fight_club.jpg" />
    <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;

namespace JQSuiteASPNETExample.examples.multiselect.templates
    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" />