How to always display the hamburger menu. Even on desktop?

How to always display the hamburger menu. Even on desktop?
http://smetuk.i-comfort.org/

Hello, try to use the following CSS code.

You can add CSS code in Customize → Additional CSS section.

nav.mobile-nav {
    display: block;
}

nav.main-navigation {
    display: none;
}

Please feel free to ask any other questions that you might have.

Kind Regards, Roman.