Well.. with Qt it's pretty easy to edit images on the fly, so that's not the problem. What will cause problems is that if you just change the hue for example the highlighted button image, it will change the hue also for the "borders" of the button (in case the button png isn't placed on transparent background) and will look ugly against the background color of the current element.

Therefore I think it would cause more harm than good to implement such a thing into Theme Customizer. These kinds of things need to be done manually with some image editor where you can set the area to adjust etc.