Accessing $vuetify instance property from vuex store

For Vuetify 2.0 you can try following method. (After following Vuetify 2.0 Upgrade guide for themes)

import Vuetify from './plugins/vuetify'

export default {
  getters: {},
  mutations: {
    toggleDarkTheme(state) {
      Vuetify.framework.theme.themes.light.primary = "#424242";
    }
  }

$vuetify is an instance property hence you can access any vue instance property using

Vue.prototype.$prop

For your case

import Vue from 'vue';
export default {
  getters: {},
  mutations: {
    toggleDarkTheme(state) {
      Vue.prototype.$vuetify.theme.primary = "#424242";
    }
  }
};