Transparent header

Hi there

I love this theme! But I can’t figure out how to change the opacity of my header in such a way that my slider, site title and menu merge. Like airbnb.com and others.
In other words. I don’t want a header with menu and title on a white background, I want that background to be the top part of my “Smart Slider”/page.

Jesus, did that even make sense?

btw
I’m new to wordpress (so bear with me, and if there is a way to edit this in custom CSS please share. Cause I cannot code.), and here’s a link to my temporary site:
http://randersfjordferietest.co.nf/da/forside/