Help with fixing the ‘masthead’ on the mobile version.

Hi,

I’m not really an experienced website builder, but recently started with it on my new website www.overdedatum.org
I really love this theme, the only problem I have is that when on the mobile version of the site, my ‘masthead’ collapses (is this the right word?) in a really weird way. My logo is way above the menu button.
I hope someone can help me with my problem, the answer is probably really simple, but as I said, this is my first time trying to build a website.

Thanks in regard,
Noa