If you look at your build_skin.hta file in C:\Program Files\Metrostyle Customizer

you'll find Line 1998:
Set objFile = objFSO.CreateTextFile(currentPath & "custom.pal")

The function is called CustomColorPal.
This function creates a file called custom.pal which contains the palette with the selected color, used to create patter_custom.png.

Custom background color uses the same function. It is unexplainable to me that it works with background and not with tabs.

It says permission denied. So, for an unknown reason custom.pal cannot be modified or created. I can do an internet search to verify what it could mean, but I'm not a programmer. Maybe you need to run the application as administrator.