Please, review the ASPX and C#/VB.NET code tabs for implementation details.
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="default.aspx.cs" Inherits="JQSuiteASPNETExample.examples.chart.columnbar.rotateddatalables._default" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "">
<html xmlns="" >
<head id="Head1" runat="server">
    <title>jqChart ASP.NET chart control</title>        
    <!-- jQuery runtime minified -->
    <script src="/js/jquery-3.2.1.min.js" type="text/javascript"></script>    
    <!-- The javascript source file of jqChart -->
    <script src="/js/trirand/jquery.jqChart.min.js" type="text/javascript"></script>    
    <style type="text/css">    
        body, html { font-size: 80%; }    
    <form id="form1" runat="server">      
    <!-- The chart can be created declaratively in ASPX or in code-behind with C#/VB.NET code -->
        <Title Text="World's largest cities per 2008" />
                <Labels Rotation="-45" Align="Right" />
                    <trirand:AxisCategory Text="Tokyo" />
                    <trirand:AxisCategory Text="Jakarta" />
                    <trirand:AxisCategory Text="New York" />
                    <trirand:AxisCategory Text="Seoul" />
                    <trirand:AxisCategory Text="Manila" />
                    <trirand:AxisCategory Text="Mumbai" />
                    <trirand:AxisCategory Text="Sao Paulo" />
                    <trirand:AxisCategory Text="Mexico City" />
                    <trirand:AxisCategory Text="Delhi" />
                    <trirand:AxisCategory Text="Osaka" />
                    <trirand:AxisCategory Text="Cairo" />
                    <trirand:AxisCategory Text="Kolkata" />
                    <trirand:AxisCategory Text="Los Angeles" />
                    <trirand:AxisCategory Text="Shanghai" />
                    <trirand:AxisCategory Text="Moscow" />
                    <trirand:AxisCategory Text="Bejing" />
                    <trirand:AxisCategory Text="Buenos Aires" />
                    <trirand:AxisCategory Text="Guangzhou" />
                    <trirand:AxisCategory Text="Shenzhen" />
                    <trirand:AxisCategory Text="Instanbul" />                    
            <trirand:ChartYAxisSettings Min="0">
                <Title Text="Population (millions)" />                
                    <trirand:ChartPoint Y="34.4" />
                    <trirand:ChartPoint Y="21.8" />
                    <trirand:ChartPoint Y="20.1" />
                    <trirand:ChartPoint Y="20" />
                    <trirand:ChartPoint Y="19.6" />
                    <trirand:ChartPoint Y="19.5" />
                    <trirand:ChartPoint Y="19.1" />
                    <trirand:ChartPoint Y="18.4" />
                    <trirand:ChartPoint Y="18" />
                    <trirand:ChartPoint Y="17.3" />
                    <trirand:ChartPoint Y="16.8" />
                    <trirand:ChartPoint Y="15" />
                    <trirand:ChartPoint Y="14.7" />
                    <trirand:ChartPoint Y="14.5" />
                    <trirand:ChartPoint Y="13.3" />
                    <trirand:ChartPoint Y="12.8" />
                    <trirand:ChartPoint Y="12.4" />
                    <trirand:ChartPoint Y="11.8" />
                    <trirand:ChartPoint Y="11.7" />
                    <trirand:ChartPoint Y="11.2" />                    
   <script type="text/javascript">
    function formatSeriesLabel()
        return this.y;
    <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.chart.columnbar.rotateddatalables
    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" />