However there is some risk in doing this currently. If you need to make revisions to the content that necessitates adjusting the background, this seems to often trigger changes to the connections within the content, presumably as they to somehow connect to the rectangle. The connections all or mostly become “disconnected” and this is not reversible with undo. They must be re-connected one-by-one by dragging their endpoints back onto the related elements.
Frustrating!
In my case I simply want to be able to export flow charts against a white background so that they have higher contrast and integrate better into documents in external tools, e.g. Confluence.