Mobile menu not visible

Hey welti,
I took a look at your site and along with your menu not showing on mobile, your menu was about halfway in the header for tablet and it looks like you wanted to add a background-image to the header, but it was not showing right.
I corrected all of these issues with this code, Please try this code in the General Styling > Quick CSS field or in the WordPress > Customize > Additional CSS field:

@media only screen and (min-width: 767px) and (max-width: 988px) { 
.responsive #top #wrap_all .header_color .header_bg {
    background-size: cover !important; 
}
.responsive #top #wrap_all .main_menu {
    background-color: transparent !important; 
}
.responsive #top #wrap_all .main_menu {
    top: 40px !important;
    width: 50% !important; 
}
}
@media only screen and (max-width: 767px) { 
.responsive #top #wrap_all .header_color .header_bg {
    background-size: cover !important; 
}
.responsive #top #wrap_all .main_menu {
    background-color: transparent !important; 
}
.responsive #top #wrap_all .main_menu {
    top: 40px !important; 
    left: 80% !important; 
}
}

then Please clear your browser cache and check.

Best regards,
Mike