Top margin hassles

Hi there
I have a need to eliminate the top margin under the header. I've put in some code I got from the forum. It seemed to work for a couple of seconds then nothing:

//* Set page top margin to 0 *//
.one-container.site-content {
    padding: 0px;
}
.elementor-column-gap-default>.elementor-row>.elementor-column>.elementor-element-populated {
    padding: 0px;
}

Any ideas?