ASP.NET offers a number of ways to assign a theme to content pages.
<%@ Page Language="C#" StylesheetTheme="Blue" %>
Assign it globally to the entire application by adding the following instruction to the web.config file:
<pages styleSheetTheme="Blue" />
Create a Folder in the name "Blue" under App_Themes.
Place your CSS File under this (App_ThemesBlueMyCSS.css)
Refer this in the <SYSTEM.WEB> section under WEB.CONFIG.
Instead of refering the CSS in all the pages, by placing an entry in WEB.CONFIG, it automatically gets reflected in all the pages.
More information is available here.