metamask-extension/ui/components/component-library/icon/icon.scss

.mm-icon {
  --icon-size: var(--size, 16px);

  font-size: var(--icon-size);
  width: 1em;
  height: 1em;
  max-width: 1em;
  flex: 0 0 1em;
  background-color: currentColor; // inherits parent text color
  mask-size: cover;
  -webkit-mask-size: cover;
  mask-repeat: no-repeat;
  -webkit-mask-repeat: no-repeat;
  mask-position: center;
  -webkit-mask-position: center;

  // Size

  &--size-xs {
    --size: 12px;
  }

  &--size-sm {
    --size: 16px;
  }

  &--size-md {
    --size: 20px;
  }

  &--size-lg {
    --size: 24px;
  }

  &--size-xl {
    --size: 32px;
  }

  &--size-inherit {
    --size: 1em; // Inherits parent font-size

    position: relative; // Fixes vertical alignment
    top: 0.125em; // Fixes vertical alignment
  }
}