Add navigation.scss on nuxt config

nuxt.config.js

...
      { hid: 'title', name: 'title', content: 'Nuxt.js project' }
    ],
    link: [
      { rel: 'icon', type: 'image/x-icon', href: '/favicon.ico' },
      { rel: 'stylesheet', type: 'text/css', href: 'https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.0.0-beta.3/css/bootstrap.min.css' }
    ]
  },
  /*
  ** Custom SCSS
  */
  css: [
    { src: '~/assets/scss/navigation.scss', lang: 'scss' }
  ],
  /*
  ** Customize the progress bar color
  */
  loading: { color: '#3B8070' },
  /*
...