Vuetify: colors are not showing up

I found the problem. I had to wrap Vuetify components inside v-app tag.

<v-app>
  <v-btn color="success">Success</v-btn>
  <v-btn color="error">Error</v-btn>
  <v-btn color="warning">Warning</v-btn>
  <v-btn color="info">Info</v-btn>
</v-app>

Vuetify documentation says:

In order for your application to work properly, you must wrap it in a
v-app component. This component is used for dynamically managing your
content area and is the mounting point for many components.