Those % images are packaged into a .artwork file. You would have to extract that artwork file which contains about 1,800 images, thumb through them, replace the % images, repackage all images as an artwork file, and then add the artwork file back to your phone. Not an easy task, but not impossible either.
If you're a Winterboard user you should be able to theme it. The images are named:
1. ColorOnGrayShadow_Percent_%@2x.png
2. ColorOnGrayShadow_PercentLow_%@2x.png
3. WhiteOnBlackEtch_Percent_%@2x.png
4. WhiteOnBlackEtch_PercentLow_%@2x.png
5. WhiteOnBlackShadow_Percent_%@2x.png
6. WhiteOnBlackShadow_PercentLow_%@2x.png
(^These are for an iPhone 4S on iOS 5.1.1. May be different for other devices and OS's.)
Create a folder (name it anything) and inside place another folder named "UIImages" (The two I's are intentional.)
Inside that place your new icons and drop the entire folder into your Themes.xxxx folder then activate in winterboard and respring.
(That's all a shot in the dark on my part, someone correct me if I'm wrong, I've never themed these images before.)