Full background CSS in PG

[Heautontimoroumenos]

Hi all
I'm building new site with PG (Foundation) and I would like to add a full background on the page. I try to use this piece of code
html {
background: url(images/bg.jpg) no-repeat center center fixed;
-webkit-background-size: cover;
-moz-background-size: cover;
-o-background-size: cover;
background-size: cover;
}
But no matter how hard I try it is not working. I use local images. Can anyone help me with this one? Is there an option to set it purely in PG not touching source code?
Best
Jimp