How to add background image to the site?

Hello, thanks for this theme, it's super useful!

My question is how to add a background image (transparent) to the hugo site? Is there someplace where we can modify to add some css code such as:

```
body {
 background-image: url("paper.png");
 background-color: #cccccc;
}
```
thanks for your help/suggestions in advance!

-- ipstone