In windows 7 ,you can change individual parts of a theme (the pictures , colors, and sounds), and then save the revised theme for your own use or to share with other people.Below is a simple guide about how to customize theme in windows 7
1.Open Personalization by right-click your desktop
2.Click the theme that you want to change to apply it to your desktop.
3.Customize some items you want to change,eg:background,color,sound ,or screen savers
Tip:You can add more pictures as your background by holding on Ctl key
4.Right-click Unsaved Theme to save or share your theme with your friends