How to add a background color to any UI element

Not all UI elements have a background color property so here is a quick way to add it to any element (or part of element):

1. Add a Rectangle element (hold R and drag) and make it borderless from the selection toolbar (or the Properties panel, if you've turned it on)
2. Set the background color to the desired one
3. Set its opacity to 25%
4. Move and resize the rectangle to overlay your other element (table in this case)
5. Group the UI elements together

You can also set up a border color the same way if needed.

The same trick works for the board itself: there's no board or page background setting (and no fixed canvas size), so draw a large borderless rectangle, give it a color, and send it to the back.