For a fully transparent background use:
background: transparent;
Otherwise for a semi-transparent color fill use:
background: rgba(255,255,255,0.5); // or hsla(0, 0%, 100%, 0.5)
where the values are:
background: rgba(red,green,blue,opacity); // or hsla(hue, saturation, lightness, opacity)
You can also use rgba values for gradient backgrounds.
To get transparency on an image background simply reduce the opacity of the image in an image editor of you choice beforehand.