I have an idea to create Themes for Android Mobile. But I have no knowledge of how to do this. I would need to know about the following things:
I would start with Google's page on Styles and Themes. Many of your questions are answered there.
http://developer.android.com/guide/topics/ui/themes.html
Here's a simple tutorial:
http://developerlife.com/tutorials/?p=309
EDIT:
If you are talking about themes for the phone instead of the app, you can create themes for specific home screen replacements such as aHome. I'm sure if you look around their site you can see how to create a theme for their app.
There are also sites like Android Themes, but they require that you have rooted your phone and have installed a custom ROM such as those by Cyanogen.
Perhaps you could also write your own custom home screen.