metamask-extension/ui/app/components/currency-input/index.scss

.currency-input {
  &__conversion-component {
    font-size: 12px;
    line-height: 12px;
    padding-left: 1px;
  }

  &__swap-component {
    flex: 0 0 auto;
    height: 24px;
    width: 24px;
    background-image: url("images/icons/swap.svg");
    background-size: contain;
    background-repeat: no-repeat;
    cursor: pointer;
    opacity: .4;

    &:hover {
      opacity: .3;
    }

    &:active {
      opacity: .5;
    }
  }
}