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.area.basic._default" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<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%; }    
    </style>
</head>
<body>
    <form id="form1" runat="server">      
    
    <!-- The chart can be created declaratively in ASPX or in code-behind with C#/VB.NET code -->
    <trirand:JQChart 
        runat="server" 
        Width="600"
        Height="410"
        Type="Area"        
        ID="BasicAreaChart">       
        <Title Text="US and USSR nuclear stockpiles"></Title>
        <SubTitle Text="Source: <a href='http://thebulletin.metapress.com/content/c4120650912x74k7/fulltext.pdf'>thebulletin.metapress.com</a>" />
        <XAxis>
            <trirand:ChartXAxisSettings>
                <Labels Formatter="formatXAxisLabel" />
            </trirand:ChartXAxisSettings>
        </XAxis>
        <YAxis>
            <trirand:ChartYAxisSettings>
                <Title Text="Nuclear weapon states" />
                <Labels Formatter="formatYAxisLabel" />
            </trirand:ChartYAxisSettings>            
        </YAxis>
        <ToolTips Formatter="toolTipFormatter" />
        <PlotOptions>
            <Area PointStart="1940">
                <Marker Enabled="false" Symbol="cicrcle" Radius="2" />
            </Area>
        </PlotOptions>
        <Series>
            <trirand:ChartSeriesSettings Name="USA"> 
                <%--Data comes from code-behind. Check the C#/VB.NET code tab for details               --%>
            </trirand:ChartSeriesSettings>
            <trirand:ChartSeriesSettings Name="Russia">                
                <%--Data comes from code-behind. Check the C#/VB.NET code tab for details               --%>
            </trirand:ChartSeriesSettings>
        </Series>
    </trirand:JQChart>
    
     <script type="text/javascript">
        
        function formatXAxisLabel()
        {
            return this.value; // clean, unformatted number for year
        }
        
        function formatYAxisLabel()
        {
            return this.value / 1000 +'k';
        }
        
        function toolTipFormatter()
        {
            return this.series.name +' produced <b>'+
               Highcharts.numberFormat(this.y, 0) +'</b><br/>warheads in '+ this.x;
        }
        
      </script> 
        
    
    <trirand:codetabs runat="server" id="tabs"></trirand:codetabs>  
    
    </form>
</body>
</html>
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.chart.area.basic
{
    public partial class _default : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            if (!Page.IsPostBack)
            {
                BasicAreaChart.Series[0].Data = new ChartPointCollection().FromCollection(usaData);
                BasicAreaChart.Series[1].Data = new ChartPointCollection().FromCollection(russiaData);
            }
        }
        int? [] usaData = new int? [] { 
            null, null, null, null, null, 6 , 11, 32, 110, 235, 369, 640, 
            1005, 1436, 2063, 3057, 4618, 6444, 9822, 15468, 20434, 24126, 
            27387, 29459, 31056, 31982, 32040, 31233, 29224, 27342, 26662, 
            26956, 27912, 28999, 28965, 27826, 25579, 25722, 24826, 24605, 
            24304, 23464, 23708, 24099, 24357, 24237, 24401, 24344, 23586, 
            22380, 21004, 17287, 14747, 13076, 12555, 12144, 11009, 10950, 
            10871, 10824, 10577, 10527, 10475, 10421, 10358, 10295, 10104 };

        int? [] russiaData =  new int? [] { 
            null, null, null, null, null, null, null , null , null ,null, 
            5, 25, 50, 120, 150, 200, 426, 660, 869, 1060, 1605, 2471, 3322, 
            4238, 5221, 6129, 7089, 8339, 9399, 10538, 11643, 13092, 14478, 
            15915, 17385, 19055, 21205, 23044, 25393, 27935, 30062, 32049, 
            33952, 35804, 37431, 39197, 45000, 43000, 41000, 39000, 37000, 
            35000, 33000, 31000, 29000, 27000, 25000, 24000, 23000, 22000, 
            21000, 20000, 19000, 18000, 18000, 17000, 16000};
                                
    }
}
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" />