It doesn't seem possible to make the bag background opaque even when I move the slider to the top I can still see through the background? I would be great to have it opaque as I find it looks messy seeing the other windows behind it.

Investigating this a bit more, it seems like it does work, but there's a bug where the texture picker doesn't update the bag frame without a UI reload.

Can you try:

1. Select Solid as your background color
2. Close the configuration window
3. Type /reload into your chat window and hit enter
4. Set the opacity to 100

It should be completely black/solid. Does that work for you?

I'm pushing a fix so the skin changes happen after you click them without having to wait for a reload. I've tested this on all versions of WoW, and it works, including your test case.

I'm able to reproduce this, marking this as a bug as this does seem like an unexpected UX.