Please have a look at https://codepen.io/vadikom/pen/pbqyOB and at https://codepen.io/vadikom/pen/jqooKo.
If You klick in Firefox(portable developer edition is at sourceforge) with the right mousekey at the right side of the nav and then choose ‘Inspect element ‘, then You see <nav role=navigation> which is also the marked in firefox.
If You add a class to <nav … > then You can style it – evtl. with !important; at the end of the specific parameter like max- height.
The next line is <ul id=main-menu class=”sm sm-simple” …> . If You mark this line You will find: ul, .sm li {.. line-height etc;
These parameters are in the sm-simple.css . Here You can change these and some other parameters.