This is the Modified Tiffany Theme ( aka Reverse Tiffany).
It's almost ready. It just needs some small refinements.
In the image attached you can see the Full Tiffany to the right of the actual screenshot of the Modified Tiffany theme . I'm beginning to comprehend what the data in the config file means. In the bar area of the config BarPre & BarPreGadget are the same value. They represent the left end cap of the TitleBar we wish to draw. The values are 0 4. But what does it mean? It's offset & length. At offset 0 copy 4 pixels.
We must first realize the values in bar section of the config text file in System inside our theme are not the co-ords we see on screen. These are the offsets & lengths as measured from the original Titlebar graphic located in System/Titlebar.
For the transition area just to the left of the TitleBar gadgets I got lucky. I measured the size of the transition in Paint as 21 x 29 pixels. I assumed it would cover at least two gadgets after the transition so I gave it another 45 pixels as fluff. So 21 + 45 = 67. Since the TitleBar graphic is only 268 x 29 pixels the transition starts at offset 201 & it extends at least 21 pixels.
In the config we see BarJoinBG which is the tileable area just before the right side gadgets. It just happens to be BarJoinBG = 200 40. Remember it's offset & length. So at offset 200 copy 40 pixels to underlay the transition area just before the end gadgets. That's perfect. Lastly, whether the transition curves one way or another it doesn't matter. We only care about the horizontal length of the transition area. So now we are ready to design an AROS Full Tiffany Theme.
But there's more to learn about the config & window themes.