<div id="app">
  <span class="bg"></span>
  <v-app id="inspire">
    <v-toolbar dark color="primary">
      <v-toolbar-side-icon></v-toolbar-side-icon>
      <v-toolbar-title class="white--text">Title</v-toolbar-title>
    </v-toolbar>
  </v-app>  
</div>

.bg {
    width: 100%;
    height: 100%;
    position: absolute;
    top: 0;
    left: 0;
    background: url( 'https://cdn.pixabay.com/photo/2016/03/26/13/09/organic-1280537_1280.jpg') no-repeat center center;
    background-size: cover;
    background-color: red;
    transform: scale(1.1);
  }

new Vue({
  el: '#app',
  
})