Here is an article from Microsoft which will tell you how to create a custom desktop theme in 7 easy steps. The 7 steps are

1. Get to know themes

2. Change the desktop background

3. Choose your favorite photos

4. Change window borders

5. Change the way your theme sounds

6. Personalize your screen saver

7. Save your theme

Read more and create your own windows 7 theme - Article here

You can see few custom Windows 7 themes Here