title: Accessing $vuetify instance property from vuex store
body: <p>I was using <code>vuetify</code> and wanted to change theme from <code>vuex</code> store using <a href="https://vuetifyjs.com/en/style/theme" rel="noreferrer">$vuetify</a> instance but i got this error <code>Cannot set property 'theme' of undefined"</code></p>
  
  <p>here is my code </p>
  
  <pre><code>export default {
    getters: {},
    mutations: {
      toggleDarkTheme(state) {
        this.$vuetify.theme.primary = "#424242";
      }
    }
  };
  </code></pre>