Thank you, I have been able to do it this way:

.uk-sticky-placeholder {height:100px !important} /* setting a standard value for all window sizes */
@media screen and (max-width: 1120px){
.uk-sticky-placeholder { height:80px !important; }
.uk-navbar { height:80px !important; }
.uk-navbar-toggle { height:80px !important;}
.uk-navbar-content { height:80px !important;}
#menu-outer { height:80px !important;}
}

The only annoying this is that there is a thin, light gray line appearing above the image slider at the smaller window sizes which is not there when it is full screen or large enough. Do you know if there is any additional element or setting which causes this?

Hi there,
I dont know where the line comes from.
You can analyze your css in this way