// Buttons

.btn {
  border: $border-width solid transparent;
  border-radius: $border-radius;
  display: inline-block;
  font-weight: 700;
  line-height: normal;
  padding: .75em 1em;
  vertical-align: middle;

  &:hover,
  &:focus {
    box-shadow: inset 0 0 0 50em $darken-2;
    border-color: rgba($black, .25);
  }

  &:active {
    box-shadow: inset 0 0 0 50em $darken-3;
    border-color: $darken-3;
  }
}

.btn-outline {
  border-color: currentColor;

  &:hover,
  &:focus {
    border-color: currentColor;
  }
}

// Buttons

.btn {
  border: $border-width solid transparent;
  border-radius: $border-radius;
  display: inline-block;
  font-weight: 700;
  line-height: normal;
  padding: .75em 1em;
  vertical-align: middle;

  &:hover,
  &:focus {
    box-shadow: inset 0 0 0 50em $darken-2;
    border-color: $darken-2;
  }

  &:active {
    box-shadow: inset 0 0 0 50em $darken-3;
    border-color: $darken-3;
  }
}

.btn-outline {
  border-color: currentColor;

  &:hover,
  &:focus {
    border-color: currentColor;
  }
}

Use variable for button hover/focus border colour
Use variable for button hover/focus border colour
