This snippet should work, for both the Edit Mode (.cm-header-1), and the Preview Mode (.markdown-preview-view h1). However, it does not work in Edit mode. Maybe someone else can add what is needed.
I played around with it a bit and I did get it to pad or margin the left side, so I know its possible, but for some reason the Top pad or margin will not work in Edit Mode.
.cm-header-1, .markdown-preview-view h1 {
padding-top: 80px !important;
}