How to change color of system box

I try to change the background color, but I must be doing something wrong, because it’s not changing.
I can select a use case, and set it to a gradient color scheme of ocean, then set that as the default for all objects (which includes the system objects). But I can’t select the system object, and right click to change the fill color. It only has background color as an option, and this isn’t working for me.
Any ideas?

Never mind. I had to click the edge of the system box first, then right click to find the fill color.