vue3中只能去修改popper-class这个属性去修改 （自己想要的样式）重点 :样式修改不能写在scoped中，可以单独开个style

  popper-class="blueBack"（blueBack为自定义class名）

 <el-select popper-class="blueBack"><el-option /></el-select>

 单独写的css 样式(不单独写不生效)

<style lang="less">
.el-popper.is-light.blueBack {background-color: #254277 !important;border: 1px solid #254277 !important;
}.blueBack.el-popper[data-popper-placement^=bottom] .el-popper__arrow::before {background: #254277 !important;border: 1px solid #254277 !important;
}.blueBack.el-popper[data-popper-placement^=top] .el-popper__arrow::before {background: #254277 !important;border: 1px solid #254277 !important;
}.blueBack {.el-select-dropdown__item.hover,.el-select-dropdown__item:hover {background: #2A77C6 !important;}.el-select-dropdown__item {color: #B3BCCE !important;}.el-select-dropdown__item.selected {background: #2A77C6 !important;}
}

  修改下拉框的边框

  :deep(.el-input__wrapper) {background: linear-gradient(90deg, #0b3571, rgba(11, 53, 113, 0) 100%);box-shadow: 0 0 0 0 var(--el-input-border-color, var(--el-border-color)) inset;}:deep(.el-select:hover:not(.el-select--disabled) .el-input__wrapper) {box-shadow: 0 0 0 0 var(--el-select-input-focus-border-color) inset !important;}:deep(.el-select .el-input.is-focus .el-input__wrapper) {box-shadow: 0 0 0 0 var(--el-select-input-focus-border-color) inset !important;}:deep(.el-select:hover:not(.el-select--disabled) .el-input__wrapper) {background: #0b3571;}:deep(.el-select .el-input__wrapper) {background: #0b3571;}

效果展示