HI!

The best way is to use cover style for mobile. 

You can: 

1. Add any custom class to the row, were you need to change background for example "custom-bg-1". (screen 2818)

2. Use next css if you want to change bg:

@media (min-width: 768px) {
.custom-bg-1 .fullwidth-block .fullwidth-block-background {
    background-image: url('URL_OF_THE_IMAGE');
}
}

3. For adding background color:

@media (min-width: 768px) {
.custom-bg-1 .fullwidth-block .fullwidth-block-background {
  background-image: none !important;
    background-color: #000;
}
}