Hi Jenny,
Thanks for writing in! You need to switch to an inline header layout instead of stacked. Then if you want the menu to be transparent and overlap you will need to use CSS.
We can assist with that, but it would be best if you already have your header set to inline, so do that first.
Appearance > Customize > Header > Logo and Navigation > Layout >> Set to Inline.
Let us know how that goes. Hope this helps – thanks!