Thank you all for the feedback and input. Fun.
I do have Affinity Photo as well, but would prefer to achieve this simple task within a single application.
I find that I frequently want to add a background to a Designer document.
I was hoping for an "add fill layer" option, but from the above comments, it appears that the AD-only approach is:
Open Document Setup
Make note of the document dimensions
Add new layer
Add a Rectangle
In the transform panel, set x and y to zero, width and height to match document dimensions
Is that accurate?