hugo-theme-introduction/assets/vendor/fontawesome-free/scss/_stacked.scss

// stacking icons
// -------------------------

.#{$fa-css-prefix}-stack {
  display: inline-block;
  height: 2em;
  line-height: 2em;
  position: relative;
  vertical-align: $fa-stack-vertical-align;
  width: $fa-stack-width;
}

.#{$fa-css-prefix}-stack-1x,
.#{$fa-css-prefix}-stack-2x {
  left: 0;
  position: absolute;
  text-align: center;
  width: 100%;
  z-index: var(--#{$fa-css-prefix}-stack-z-index, #{$fa-stack-z-index});
}

.#{$fa-css-prefix}-stack-1x {
  line-height: inherit;
}

.#{$fa-css-prefix}-stack-2x {
  font-size: 2em;
}

.#{$fa-css-prefix}-inverse {
  color: var(--#{$fa-css-prefix}-inverse, #{$fa-inverse});
}