How to use vuetify themes?

Question

Vue.js

In main.js

Vue.use(Vuetify, {
  theme: {
    primary: '#3f51b5',
    secondary: '#b0bec5',
    accent: '#8c9eff',
    error: '#b71c1c',
    wraning: 'red'
  }
});

In any component I try to use this theme like this: Why does the theme not work? On the site, the documentation was checked many times, nothing was fixed. help me please
<v-btn color="primary">Button</v-btn>

Answer 1

Has the tag v-appbeen added to the application root?