I love the windowskin!!
So the actual error itself isn't even the text alignment.
Your windowskin's border is exceeding 32 which is why it 'SEEMS' off. Truthfully, the text is dsplayed in the proper location. I think you may have to take into consideration, that if you alter the default X location of the text, then that means that the text will overflow out the other side of the window.
If you change the X, then you are going to have to be careful with how much text you place on each line of your message box.
**Anyway, let me take a look at the UMS, because I also use it. I am familiar with the script, so I will see if I can find the section that needs to be edited.