If you've already completed your application, it's too late. You had to code "skinning" support into your application from the start.
You can find an exmaple of such by reading Adding custom skins for Forms in VB.Net
]. It's not exactly easy and it's a ton of work.
You can also use 3rd party libraries to do this, such as .NET Advantage by Infragistics. All of these libraries cost $$$$. I don't know of any decent free libraries. Oh, and using these libraries WILL force to you rewrite the app to support skinning.
The real question should be "does adding skinning support add any value to the application"?? That answer is usually "No".