You can add a background color by following these steps:
- Open a new board
- Choose a background colorway using Colourlovers
- Drag and drop your chosen color from the Colourlovers onto the blank board
- You now have to resize the color to fit the whole page.
- You can do this by clicking on the color and using the toggle bars on the corners, this will stretch it.
For steps how to add texture or an image as a background, see more here.