Hello!
You can do it using CSS classes.
<%@ Page Language="C#" %>
<%@ Register Assembly="Ext.Net" Namespace="Ext.Net" TagPrefix="ext" %>
<!DOCTYPE html>
<html>
<head runat="server">
<style>
.ux-custom-title-close-icon .x-tool-close {
background-image: url('http://png-5.findicons.com/files/icons/753/gnome_desktop/16/gnome_window_close.png');
background-position: 0 0;
}
.ux-custom-title-close-icon .x-tool-over .x-tool-close {
background-image: url('http://png-5.findicons.com/files/icons/753/gnome_desktop/16/gnome_window_close.png');
background-position: 0 0;
}
.ux-custom-title-close-icon .x-window-header-text {
font-size: 20px;
}
</style>
</head>
<body>
<form runat="server">
<ext:ResourceManager runat="server" />
<ext:Window
ID="Window1"
runat="server"
Title="Hello World"
Icon="Application"
Height="185"
Width="350"
BodyStyle="background-color: #fff;"
BodyPadding="5"
Cls="ux-custom-title-close-icon"
Modal="true">
<Content>
This is my first <a target="_blank" href="http://www.ext.net/"> Ext.NET</a> Window.
</Content>
</ext:Window>
</form>
</body>
</html>