Remove white space for mobile

You can remove this by adding the following CSS code in the Custom CSS option of your theme on the below path.

Admin Area -> Appearance -> Customize -> Additional CSS

@media screen and (max-width: 56.875em){
header#masthead {
    margin-bottom: 5px;
}

main#main > article:first-child {
    padding-top: 0;
}
}