How I can overlap my menu in my header picture

by

I just did this on one of my sites today. This is what i have done on css.

header.header-class
{
background-color: rgba(255, 255, 255, 0);
}
div.main-content-className{
margin-top:-75px;
}

here change header-class and main-content-className with your theme class name and if necessary make it more margin on top until it will be on top.
Hope it helps.