Your map's background colour is the colour that sits behind the artwork. It shows wherever your image is see-through, and around the edges when the image doesn't fill the whole frame.
Where to set it
Open the map's Styles tab. Under Map colours you'll find Primary background. Click its swatch to choose a colour. There's also a Secondary background for a second surface colour. Leave either one off and the map uses your company brand instead.
When the background shows
The background is most visible when your map image has a transparent background (PNG or SVG), so an irregular building or campus sits directly on your colour with no white box. It also shows in any margin when the artwork doesn't quite fill the frame. Pick a colour that flatters the artwork and gives pins and highlights something to stand out against.
Works with transparency
Background colour and image transparency go hand in hand. For which image formats can be see-through, and when a transparent export helps or trips you up, see Transparency and the map background.