Hi Lars, give this a try:
@media screen and ( max-width: 1280px ) {
.main-navigation li a:hover, .main-navigation li a:focus, .main-navigation li a:active {
color: #bb00bb;
}
}
Change the example purple to whatever you like.
Don’t edit the theme files directly, otherwise your changes will be overwritten every time the theme is updated.
An easy way to add custom CSS is to use the CSS editor included in the Customizer as of WordPress 4.7. Head to Appearance > Customize > Additional CSS to add your custom CSS.
If you need further help, could you provide a link to your site? Thanks!