I haven’t tested it in 8.0.12 but for a long time all docked views had a default white background behind the primary root container that couldn’t be changed
In 8.0.13 they introduced a huge theming update, which chances are may allow you to do exactly what you are asking. So if it doesn’t work in 8.0.12 you could test it in the 8.0.13 release candidate
Either way the answer is:
On the style property for your root container, add backgroundColor and give it a color code with some transparency