I see, do you want the result like that https://imgur.com/bthy2Ev ?
The background parallax is correct :), however, the helium header using custom css for margin top and it’s caused the issue. Example:

@media only all and (min-width: 48rem) {
.g-home-particles.g-helium-style #g-header {
    margin-top: -9.5rem;
    padding: 0;
}}

you can disable the code above and add custom css code for navigation like so

@media only all and (min-width: 48rem) {
.g-home-particles.g-helium-style #g-navigation {
    background: transparent;
    position: absolute;
    z-index: 980;
    left: 0;
    right: 0;
}}

Thank you