I wanted to write a tutorial about it because someone else asked for an explanation too, but eventually I discovered a bug while doing so and I spend the time that I had with fixing it and finishing the tests for the Texture class instead of actually writing the tutorial.
Writing a tutorial is a boring task anyway so I'm not really motivated to write it during these few hours a week that I have some spare time now.
The values are (LEFT, TOP, width, height) but for the Middle rect it should be noted that it is relative to the Part rect (if Part rect is given).
The Part and Middle rect are both optional, to load the image from a separate file, just write the following:
NormalImage : "ThemeNormalImage.png";