popper-class="test"

<el-tooltip content="Top center" placement="top-start" popper-class="atooltip">
      <el-button>Dark</el-button>
    </el-tooltip>

.atooltip.aui-tooltip__popper[x-placement^=top] .popper__arrow{
      border-top-color: #e4f0f0 !important;
    }
  .atooltip.aui-tooltip__popper[x-placement^=top] .popper__arrow:after {
      border-top-color: #e4f0f0 !important;
    }
  .atooltip.aui-tooltip__popper[x-placement^=bottom] .popper__arrow{
      border-bottom-color: #e4f0f0 !important;
    }
  .atooltip.aui-tooltip__popper[x-placement^=bottom] .popper__arrow:after {
      border-bottom-color: #e4f0f0 !important;
    }
    .atooltip{
      background: #e4f0f0 !important;
      border:1px solid #e4f0f0 !important;
    }