The background in boost is colourless. Its almost a perfect white. Where there is the option to choose an image for a background, perhaps you just want a simple colour or gradient in the background. This short video shows how this can be accomplished using the CSS below:

/* This changes the background colour */
.pagelayout-login #page,body {
background: linear-gradient(to right,#b1e1b8,#bffdf7,#b1e1b8);
}