I am adding a couple of images to further explain this request. These images are of the Samsung e-mail widget on my tablet. They are in landscape because I use the tablet with a keyboard.
This is how it appears on the screen.
After a long press. Note that it allows the selection of settings for the widget.
The settings screen allows the selection of white or black background and the percentage of transparency.
I think such possibilities would be a great improvement for the Thunderbird widget.