Adding Skins to Themes : Themes « Development « ASP.NET Tutorial

A Theme can contain one or more Skin files. 
A Skin modifies any of the appearance properties of an ASP.NET control.

File: App_Themes\Simple\TextBox.skin

<asp:TextBox
    BackColor="Yellow"
    BorderStyle="Dotted"
    Runat="Server" />

You can name a Skin file anything you want. 

A Theme folder can contain a single Skin file that contains Skins for all controls. 
A Theme folder can contain hundreds of Skin files, each of which contains a single Skin. 
Everything in a Theme folder gets compiled into one Theme class.
You must include a Runat attribute, but you can never include the ID attribute when declaring a control in a Skin.

File: ShowSkin.aspx uses the Simple Theme.
Include a Theme attribute in its <%@ Page %> directive. 

<%@ Page Language="C#" Theme="Simple" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <title>Show Skin</title>
</head>
<body>
    <form id="form1" runat="server">
    <div>

    <asp:TextBox
        Runat="server" />

    </div>
    </form>
</body>
</html>

9.42.Themes
	9.42.1.	An ASP.NET Theme enables you to apply a consistent style to the pages.
	9.42.2.	How Themes Work
	9.42.3.	Adding Skins to Themes
	9.42.4.	Creating Named Skins with a SkinID property
	9.42.5.	Themes Versus StyleSheetThemes
	9.42.6.	Override Skin properties by applying a Theme to a page with the StyleSheetTheme attribute instead of the Theme attribute.
	9.42.7.	Disabling Themes
	9.42.8.	Adding Cascading Style Sheets to Themes
	9.42.9.	Handling Theme Conflicts
	9.42.10.	Configure specific controls so they opt out of the theming process entirely.
	9.42.11.	Share the Theme among multiple web applications running on the same web server
	9.42.12.	Applying Themes Dynamically
	9.42.13.	Applying Skins Dynamically
	9.42.14.	Show Dynamic CSS