Simply put, what's the best way to simply add a store-wide background image in the RDW theme ?

I'm using the default RwD theme in Magento 1.9.2, and I can't seem to figure out how to just add an image, not insert it in CMS, not a block, just a static background image on the whole page instead of the default white colour.

I've been trying to add it, like you usually would, in styles.css

```
body {
    margin: 0;
    text-align:center; 
    font-family:futura;
    background-image:url(../images/Fond-Portail.jpg);
    background-size: 100% 100%; 
    background-position: center top; 
    background-attachment: fixed;
}

```

The URL should be correct, this is usually a very simple task but I think RWD makes it harder. The only solutions I've seen to this problem, while searching, were to use a community module, is it really that complicated or am I missing something ?