Twentytwelve – removing underlining from navigation

I have read info on this but not achieved the result. I particularly would like to remove the underlining on the main navigation when viewed on small devices – which I recreate by reducing size of pc screen.

I have added this code to child theme and deleted the cache.

a: {
text-decoration: none;
}

a:hover {
text-decoration: underline;
}

Site here – https://www.moonglass.net/piggies