Vue

HTML

<div id="app">
  <div class="bold"
       :class="colorClass">
         TEXT
  </div>
</div>

CSS

.bold {
  font-weight: bold;
}

.red {
  color: red;
}

Vue

new Vue({
  el: "#app",
  data () {
  	return {
    	colorClass: 'red'
    }
  }
})