You can add custom toolbar buttons to the grid using the ToolBarSettings.CustomButtons collection. A custom button has many properties, like Text, ToolTip, ButtonIcon,Position and OnClick client-side event handler - the name of the client-side javascript function that will fire on clicking the button. ButtonIcon is the name of the CSS class that will be applied to the button that holds image information (as per ThemeRoller specs).
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="default.aspx.cs" Inherits="JQSuiteASPNETExample.examples.functionality.custom_button._default" %>
<%@ Register Assembly="Trirand.Web" TagPrefix="trirand" Namespace="Trirand.Web.UI.WebControls" %>

<!DOCTYPE html>
<html lang="en-us"
<head id="Head1" runat="server">
    <meta charset="utf-8">
    <title>jqGrid for ASP.NET WebForms - custom toolbar buttos</title>
    <!-- The jQuery UI theme that will be used by the grid -->
    <link rel="stylesheet" type="text/css" media="screen" href="" />
    <!-- The jQuery UI theme extension jqGrid needs -->
    <link rel="stylesheet" type="text/css" media="screen" href="/themes/ui.jqgrid.css" />
    <!-- jQuery runtime minified -->
    <script src="/js/jquery-3.2.1.min.js" type="text/javascript"></script>
    <!-- The localization file we need, English in this case -->
    <script src="/js/trirand/i18n/grid.locale-en.js" type="text/javascript"></script>
    <!-- The jqGrid client-side javascript -->
    <script src="/js/trirand/jquery.jqGrid.min.js" type="text/javascript"></script>    
    <style type="text/css">    
        body, html { font-size: 80%; }    

    <form id="form1" runat="server">
    <asp:AccessDataSource ID="AccessDataSource1" runat="server" 
        SelectCommand="SELECT [OrderID], [Freight], [OrderDate], [ShipCity] FROM [Orders]">

    <trirand:JQGrid runat="server" ID="JQGrid1" DataSourceID="AccessDataSource1">        
            <trirand:JQGridColumn DataField="OrderID" PrimaryKey="True" />                                           
            <trirand:JQGridColumn DataField="Freight" Editable="true" Sortable="false" />
            <trirand:JQGridColumn DataField="OrderDate" DataFormatString="{0:d}" />
            <trirand:JQGridColumn DataField="ShipCity" />
        <ToolBarSettings ShowRefreshButton="True" ShowSearchButton="True" ToolBarPosition="TopAndBottom">                
                    Text = "Send Mail"
                    ToolTip = "Send Mail"
                    ButtonIcon = "ui-icon-mail-closed"
                    Position = "Last"
                    OnClick = "customButtonClicked"
                   Text = ""
                   ToolTip = "Send Mail"
                   ButtonIcon = "ui-icon-pencil"
                   Position = "Last"
                   OnClick = "customButtonClicked"
    <br /><br />    
    <trirand:codetabs runat="server" id="AutomaticSortingTabs"></trirand:codetabs>
    <script type="text/javascript">         
        function customButtonClicked()
            alert("You have clicked a custom button.");

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

namespace JQSuiteASPNETExample.examples.functionality.custom_button
    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" />