Change the font

ShiftNav inherits the font from your theme, so the simplest way to set ShiftNav’s font is just to set a font on the body

body{
  font-family: Helvetica, sans-serif;
}

You can also target ShiftNav specifically

.shiftnav{
  font-family: Helvetica, sans-serif;
}

However, sometimes a theme or another plugin will add styles that affect the menu (for example, for .menu-item), so you may need to add more specific selectors to override the font in those cases.