Adding Cascading Style Sheets to Themes : Themes « Development « ASP.NET Tutorial

If you add a CSS file to a Theme folder, then the CSS is applied to every page to which the Theme is applied.

File: \yourApplicationRoot\App_Themes\StyleTheme\SimpleStyle.css

.content
{
    margin:auto;
    width:600px;
    border:solid 1px black;
    background-color:White;
    padding:10px;
}
.button
{
    background-color:#eeeeee;
}

            
File: ShowSimpleCSS.aspx

<%@ Page Language="C#" Theme="StyleTheme" %>
<!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 id="Head1" runat="server">
    <title>Show Simple CSS</title>
</head>
<body>
    <form id="form1" runat="server">
    <div class="content">

    <h1>Registration Form</h1>

    <asp:Label
        id="lblFirstName"
        Text="First Name:"
        AssociatedControlID="txtFirstName"
        Runat="server" />
    <br />
    <asp:TextBox
        id="txtFirstName"
        Runat="server" />

    <br /><br />

    <asp:Label
        id="lblLastName"
        Text="Last Name:"
        AssociatedControlID="txtLastName"
        Runat="server" />
    <br />
    <asp:TextBox
        id="txtLastName"
        Runat="server" />

    <br /><br />

    <asp:Button
        id="btnSubmit"
        Text="Submit Form"
        CssClass="button"
        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