While you could include the global app styles as part of the app component's non-scoped styles, Vue CLI-based projects support importing CSS files in code:

1. Create a new file /src/styles.css
2. Write the following CSS:

html {
  box-sizing: border-box;
  font-size: 62.5%;  /* =10px */
}
 
*, *:before, *:after {
  box-sizing: inherit;
}
 
html, body {
  margin: 0;
  padding: 0;
  width: 100%;
  height: 100%;
}
 
body {
  font-size: 1.4rem;
}

.hidden {
  display: none !important;
}
 
.maxHeight {
  height: 100%;
}
 
.maxWidth {
  width: 100%;
}
 
.flex {
  display: flex;
}
 
.flex-justify-content--center {
  justify-content: center;
}
 
.flex-align-items--center {
  align-items: center;
}

3. Import the styles in main.js, as shown in the following code:

import Vue from 'vue';
import App from './App.vue';
import './styles.css';
 
Vue.config.productionTip = false;
 
new Vue({
  render: h => h(App)
}).$mount('#app');