Vue 实现v-for循环的时候更改 class的样式名称

data(){
      return{
        colorList:['primary','danger','secondary','info']
      }
    },
    methods:{
      calculate(index){
        var nm = this.colorList[Math.floor(Math.random() * this.colorList.length)];
        return "card mb-3 col-lg-3 border-"+nm;
      }
    }

    <div v-for="(i,a) in serve" class="sever_box2">
      <div :class="sstt[a]">
       <img :src="i.imgs" alt=""/>
       <router-link :to="i.url">
        <span>{{i.title}}</span>
       </router-link>
       <p>{{i.english}}</p>
      </div>
     </div>

data() {
   return {
    sstt: [
     "ss1",
     "ss2",
     "ss3",
     "ss4",
    ]
   }

.ss1{
  background: #71b262;
 }
 .ss2{
  background: #6288b2;
 }
 .ss3 {
  background: #ecac60;
 }
 .ss4{
  background: #f87171;
 }