Vue

HTML

<div id="app" :class="selected">
  <select v-model="selected">
    <option v-for="myClass in classes" :value="myClass">{{ myClass }}</option>
  </select>
</div>

CSS

body {
  background: #20262E;
  padding: 20px;
  font-family: Helvetica;
}

#app {
  border-radius: 4px;
  padding: 20px;
  transition: all 0.2s;
}

.blue {
  background: blue;
}

.red {
  background: red;
}

.green {
  background: green;
}

Vue

new Vue({
  el: "#app",
  data: {
  	selected: 'blue',
    classes: [ 'blue', 'red', 'green'
    ]
  }
})