So, Update. I was able to get myself far more working space for individual elements by defining them as separate images in the gfx.xml file.
Like so. This let me create an entirely different png for each piece I wanted. However, an issue I've come across is the fact that the elements seem to be locked to specific positions.
In Battle.xml you can find this set of code that defines the borders for the player's health bar background. The usual formatting for this would be x,y,w,h however, when you edit the settings themselves it doesn't seem to make sense.
Editing the latter two values changes the size of your image, and by my assumption scales it to your set border.
However, editing the former two values also does the same.
I originally assumed that this was because the first two values take an x and y input and stretches the image to that given location, while the w and h inputs would determine how much of the image is used.
This doesn't seem to be the case though, since:
1. Positive values for the first two numbers stretch the image to the bottom right direction, while negative values reset the image to its original size.
2. Any values entered into the second set of values ends up doing just about the same thing, identically.
It seems like the 0,0 co-ordinate of the referenced image is fixed in a position, and the image scales off of that.
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At this point I've hit a roadblock. I'm able to both replace and resize elements individually, however repositioning it and its intricacies are seemingly beyond me at the moment.
If anyone has any input they could share or something they've found that would help, I'd appreciate it.
Also, I have tried Gilan's idea of changing the xywh bounds, which doesn't seem to work as far as I've tried. I may have done it wrong, but when I extend the png for the healthbar, alongside its parameters, the health bar completely disappears from the GUI. I've also attempted to increase the borders for these elements in the Batlle.xml file, but it doesn't seem to do a thing either.
Overall, I think being able to use separate PNGs could be useful to other people when making Themes. However, in order to make something unique, the ability to reposition would be necessary. I'll try to keep digging into things.