You need to use single quotes around your url.
Like this:
#content {
background: url('http://www.smoothjazzrx.com/html/wp-content/themes/graphene/images/uploads/content-bg.jpg') repeat;
padding-top: 21px;
padding-bottom: 20px;
}
You definitely want to create a child theme if you are playing around with css customization.
You can find out more either here:
https://forum.graphene-theme.com/graphene-support/how-to-modify-the-theme-using-a-child-theme
Or here:
It’s really not too difficult.