GtkNotebook {
     background-color: #e9e9e9;
 }

just for future reference: seems like css node GtkNotebook has been
deprecated/removed (i think starting with 3.19.7). 'notebook' is the
replacement, but for whatever reason its background-color does not
propagate down... to change the main child area from a white background
to e.g. a gray one, this works:

notebook stack {

 background-color: #ededed;

}

/det