As default the background layer for the thumbs and the Bing map are the same.
It may make sense for the thumbs to use a black alpha 0.5 but not for the Maps if you want to have it placed with 30% width at the side.
I can not find any easy way to change this as there is no reference at all from the map or the thumbs to the container layer.
Another thing is that the width and height of the map does not respond as I expect. Default setting is height 50% but it actually opens at around 90% and changing height does not change anything.
Only width does react as expected.
Hans