By default, top section in Squarespace will have a default padding.
See screenshot, green is top default padding.
On each screen size, padding is different. So to reduce space between Header and Content, you will need to set CSS code on different screen sizes.
Use this code to Custom CSS box
/* Desktop */
@media screen and (min-width:992px) {
#page>article>section:first-child {
padding-top: 50px !important;
}
}
/* Tablet */
@media screen and (min-width:768px) and (max-width:91px) {
#page>article>section:first-child {
padding-top: 30px !important;
}
}
/* Mobile */
@media screen and (max-width:767px) {
#page>article>section:first-child {
padding-top: 90px !important;
}
}