@Ricky_Turner did something similar with his custom pool dashboard. From what I understand, he roughly laid out the tiles on his dashboard, then took a screenshot of that and drew a background to match the placement of the tiles.
In Ricky’s case, I believe the tiles all fit on a single screen, so there is no scrolling… so the image is the size of the screenshot. If you have tiles that flow beyond the screen size, the background will scale to fit the full dashboard size (eg. Including the height of the content that would scroll below the screen).