metamask-extension/ui/components/app/advanced-gas-fee-popover/index.scss

.advanced-gas-fee-popover {
  &__wrapper {
    border-top: 1px solid $ui-grey;
  }

  &__separator {
    border-top: 1px solid $ui-grey;
    margin: 24px 0 16px 0;
  }
}