.null-input .el-input__inner {
background-color: rgba(255, 255, 255, 0.8); 
color: #525661; 
font-size: 16px;
}

.no-null-input .el-input__inner { 
background-color: rgba(255, 255, 255, 1); 
color: #524F52; 
font-size: 16px;
}

:class="{ 'null-input':isNullInput, 'no-null-input':!isNullInput}" @change="changeCss"

changeCss(val) { 
if (val === "") { 
this.isNullInput = true; 
} else { 
this.isNullInput = false; 
} 
console.log(this.isNullInput); 
}