Using breakpoints

I am trying to use a different home background image on phones using media queries, but it doesn’t seem to work. Can you please help?

@media only screen and (max-width: 479px) {

body.home .header-image {

background-image: url(https://domain/wp-content/uploads/2017/09/home-phone-bg1-1.jpg) !important;
display: block;
height: auto !important;

}