With the Classic Designer, we were able to set our background image (a picture of our main office) to fill the available space with the following CSS:

/*This is useful when using pictures for a background*/
body {
  background-repeat: no-repeat; 
  background-position: center;
  background-attachment: fixed;
  background-size: cover;
}

I've tried adding this CSS to the theme of the new Designer, but it does not work; it remains tiled per theme settings.  So I suppose the first question is, why isn't there an option to stretch the image to fill, instead of just tiling?  Secondly, in the meanwhile, is there any way to apply that background css in the New Designer?