There are many people who do not happy with large (4px) windows borders in Windows 8 . As you may know, Microsoft has removed the classic appearance feature from Windows 8 and you not able to reduce windows borders anymore without change in registry.
Move the slider to left if you want to disable huge windows borders in Windows 8, or set any border size from 1 to 10px as you like. Click "Apply" and you are done!
-Live preview is removed to improve the speed.