Adding theme can be done with the help of following steps
step 1:
From the solution explorer window select your site and then right click to add ASP.NET Folder you will easily see it and from their select the last option thats the theme folder and give it a name say Radix
Step 2:
Now select the Theme folder in this case Radix and right click on it to add the skin file, when the skin file opens you will see all the comments remove those comments and add the following snippet of code which will change the forecolor of a single textbox to red in a single page
<asp:textbox runat="server" forecolor="Red" skinid="1" xmlns:asp="#unknown"></asp:textbox>
Step 3:
Select source view and from their add Theme attribute in the page directive after you have add the theme directive your directive will look like this
<<%@ Page Language="C#" AutoEventWireup="true" Theme="Radix" CodeFile="Default.aspx.cs" Inherits="_Default" %>>
Step 4:
Go to the design view drag n drop a textbox and from their select its skinid to 1 and then run your application and you are done
For more information on themes search
Google
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Thanks & Regards
Radix :)