Okay Leo, I found the main issue. There was another simple-css code of line still in the post. Which line can I remove to adjust it for mobile devices?
/* remove space between nav bar and content */
@media (max-width: 768px) {
/* CSS in here for mobile only */
.separate-containers .site-main, .entry-content {
margin-top: 0px;
}
.separate-containers .inside-article {
padding-top: 0px;
}
}