How to Create Your Own Windows 7 Themes
1. Right Click on your Desktop and Select Personalize from Context Menu
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2. Now click on “desktop background” to change background wallpaper
3. Click Browse to locate folder of high resolution wallpapers
4. Select check images/wallpapers if you want to have slideshow feature in your theme
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5. Click on save changes.
5. Now click Windows color to change color of windows, background, and cursor
6. Click on sound to change system sound default and change screen saver.
8. Once you have finished your customization click on save theme option in your personalization window.
9. Enter theme name and select themepack as save type
That's it, in order to install theme just double click on themepack file.
Here are a few of my favorites already created from Microsoft