@Uvah- If you just want to see the differences in settings, an easy way might be to create a new UIS1 skin and a new UIS2 skin. SkinStudio, "New skin" at left. "Create new skin" window pops up.
"WindowBlinds basic skin" (1st item) is a UIS1 skin. I see here too it says "They are the most compatible type of skin, but are more limited." That might affect the Thunderbird app in OP.
"WindowBlinds advanced skin" (2nd item) is a UIS2 skin.
While poking around I also see that this is wrong:
If there aren't any UIS2 frames defined it will use a default, I think Diamond frames
I think I'm used to seeing skins where Diamond was used as a base and not changed. Kind of looks like something has to be defined.
Also looks like a person might be able to make a UIS1 skin and add per pixel frames to it, not sure.
Good hunting Uvah.