How It Works
In this exercise you first applied the Monochrome theme by changing the element in the Web.config file. When the run time sees that a theme is active, it scans the associated theme folder for .css files and includes a link to all those files in the page’s section in alphabetical order. In the case of the Monochrome theme it finds the file Monochrome.css and adds it to the section automatically.
An identical process took place when you changed the theme to DarkGrey. The linked style sheet then influences the way the page is displayed in the browser by changing the layout and colors used in the page.
To enable design-time support in Visual Studio, you need to change the styleSheetTheme in the Web.config file as well. The only downside of this is that the relevant CSS file is now included twice: once for the Theme and once for the StyleSheetTheme. Because the exact same file is included twice, it doesn’t affect the layout of the site. All the selectors in the second file simply overrule those in the first. However, if you feel this duplication is a waste of CPU cycles, you should delete the styleSheetTheme attribute from the Web.config file when you go live with the application.
The layout of the page is changed radically because of the CSS in the DarkGrey.css file. If you want to
know what CSS the file contains and what elements of the page it changes, open it up in VS. It has lots of comments describing each selector in detail.