To define Theme you have to create first page theme and then you have to create
skin file.
Steps for creating Page Themes
1. In the Solution Explorer, right-click on your web site name and hover to
Add ASP.NET then click Themes.
2. Now you will see an App_Themes folder.
3. Then create Newfolder of ThemeFolder in the App_Themes .
Steps for creating Skin file
4. Right-click on the ThemeFolder and click on Add New Item.
5. Then, you will see the Add New Item dialog box, you have to click on Skin File.
6. Type the name of the .skin file in the textbox.
Now you can add following .skin file code to created .skin file
ASP.Net code to define theme :
<asp:label runat="server" Width="100px" Height="30px" Font-Size="Large"
Font-Bold="true" ForeColor="#666666" BackColor="#FFFFFF"
BorderColor="#FFFFFF" />
<asp:button runat="server" ForeColor="#000000" BackColor="#000000"
BorderColor="#FFFFFF" Font-Italic="true"/>