This CSS works for me on Forms Version 11.0.2212.30907 with the title background image set via URL from the theme details.

Classic Designer: 

#cf-formtitle {
    background-size: contain;
    background-repeat: no-repeat;
    width: 100%;
    height: 0;
    padding-top: 50%;   /* this is roughly the image height divided by image width - may need some minor tweaking for best appearance. */
}

New Designer: 

.form-header {
    background-size: contain;
    background-repeat: no-repeat;
    background-position: center;
    width: 100%!Important;
    height: 0!Important;
    padding-top: 50%!Important;   /* this is roughly the image height divided by image width - may need some minor tweaking for best appearance. */    
}

Regarding the part where it says "this is roughly the image height divided by image width - may need some minor tweaking for best appearance".  I mention the tweaking because in my case, the image height divided by image width was 51.84% - but I found it looked best on the Classic Designer at 50% and on the New Designer at 52%.