You can perform many tasks such as toggle, show, hide, select, unselect, etc on the client-side using the client-side API. To get the client-side instance (object) of the dropdownlist, get its jQuery selector using its client-side ID and call the getDropDownListInstance() method. The method will return the dropdownlist client-side instance which you can later use to call many client-side API methods.
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="default.aspx.cs" Inherits="JQSuiteASPNETExample.examples.dropdownlist.clientapi._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 - Client-Side API</title>
    <!-- The jQuery UI theme that will be used by the dropdownlist  -->
    <link rel="stylesheet" type="text/css" href="http://code.jquery.com/ui/1.12.1/themes/redmond/jquery-ui.css" />    
    <!-- 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">
    <table cellpadding="10" cellspacing="10">
            <td valign="top">
                        <trirand:JQListItem Text="One" Value="One" />
                        <trirand:JQListItem Text="Two" Value="Two" />
                        <trirand:JQListItem Text="Three" Value="Three" />
                        <trirand:JQListItem Text="Four" />
                        <trirand:JQListItem Text="Five" />
                        <trirand:JQListItem Text="Six" />
                        <trirand:JQListItem Text="Seven" />
                        <trirand:JQListItem Text="Eight" />
                        <trirand:JQListItem Text="Nine" />
                        <trirand:JQListItem Text="Ten" />
            <td valign="top">
                 <fieldset title="DropDown">
                    <legend style="font-size:120%; font-family: Tahoma">DropDown</legend>
                    <input type="button" value="Show DropDown" onclick="showDropDown()" />
                    <input type="button" value="Hide DropDown" onclick="hideDropDown()" />
                    <input type="button" value="Toggle DropDown" onclick="toggleDropDown()" />
                <br /><br />
                <fieldset title="Selection">                
                    <legend style="font-size:120%; font-family: Tahoma">Selection</legend>
                    <input type="button" value="Find by text nad select 'One'" onclick="selectOne()" />
                    <input type="button" value="Find by value nad select 'Two'" onclick="selectTwo()" />
                    <input type="button" value="Find by index nad select 'Three'" onclick="selectThree()" />
                <br /><br />
                <fieldset title="Enabled/Disable">
                    <legend style="font-size:120%; font-family: Tahoma">Enable/Disable</legend>
                    <input type="button" value="Enable DropDown" onclick="enableDropDown()" />
                    <input type="button" value="Disable DropDown" onclick="disableDropDown()" />                    
                    <br />
                    <input type="button" value="Enable 'Two'" onclick="enableTwo()" />
                    <input type="button" value="Disable 'Two'" onclick="disableTwo()" />
    <br /><br /><br />
    <br /><br /><br />
     <script type="text/javascript">
    function showDropDown() {
        var dropDown = $("#<%= JQDropDownList1.ClientID %>").getDropDownListInstance();
    function hideDropDown() {
        var dropDown = $("#<%= JQDropDownList1.ClientID %>").getDropDownListInstance();
    function toggleDropDown() {
        var dropDown = $("#<%= JQDropDownList1.ClientID %>").getDropDownListInstance();
    function selectOne() {
        var dropDown = $("#<%= JQDropDownList1.ClientID %>").getDropDownListInstance();
        var item = dropDown.getItemByText("One");
    function selectTwo() {
        var dropDown = $("#<%= JQDropDownList1.ClientID %>").getDropDownListInstance();
        var item = dropDown.getItemByValue("TheValueOfTwo");
    function selectThree() {
        var dropDown = $("#<%= JQDropDownList1.ClientID %>").getDropDownListInstance();
        var item = dropDown.getItemByIndex(2); // index is zero based, first item has index = 0
    function enableDropDown() {
        var dropDown = $("#<%= JQDropDownList1.ClientID %>").getDropDownListInstance();
    function disableDropDown() {
        var dropDown = $("#<%= JQDropDownList1.ClientID %>").getDropDownListInstance();
    function enableTwo() {
        var dropDown = $("#<%= JQDropDownList1.ClientID %>").getDropDownListInstance();
        var item = dropDown.getItemByText("Two");
        if (item)
    function disableTwo() {
        var dropDown = $("#<%= JQDropDownList1.ClientID %>").getDropDownListInstance();
        var item = dropDown.getItemByText("Two");        
        if (item)
        <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.dropdownlist.clientapi
    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" />