twbs--bootstrap/scss/mixins/_buttons.scss

// Button variants
//
// Easily pump out default styles, as well as :hover, :focus, :active,
// and disabled options for all buttons

@mixin button-variant(
  $background,
  $border,
  $color: color-yiq($background),
  $hover-background: darken($background, 7.5%),
  $hover-border: darken($border, 10%),
  $hover-color: color-yiq($hover-background),
  $active-background: darken($background, 10%),
  $active-border: darken($border, 12.5%),
  $active-color: color-yiq($active-background)
) {
  color: $color;
  @include gradient-bg($background);
  border-color: $border;
  @include box-shadow($btn-box-shadow);

  &:hover {
    color: $hover-color;
    @include gradient-bg($hover-background);
    border-color: $hover-border;
  }

  &:focus,
  &.focus {
    color: $hover-color;
    @include gradient-bg($hover-background);
    border-color: $hover-border;
    // Avoid using mixin so we can pass custom focus shadow properly
    @if $enable-shadows {
      box-shadow: $btn-box-shadow, 0 0 0 $btn-focus-width rgba(mix($color, $border, 15%), .5);
    } @else {
      box-shadow: 0 0 0 $btn-focus-width rgba(mix($color, $border, 15%), .5);
    }
  }

  // Disabled comes first so active can properly restyle
  &.disabled,
  &:disabled {
    color: $color;
    background-color: $background;
    border-color: $border;
    // Remove CSS gradients if they're enabled
    @if $enable-gradients {
      background-image: none;
    }
  }

  &:not(:disabled):not(.disabled):active,
  &:not(:disabled):not(.disabled).active,
  .show > &.dropdown-toggle {
    color: $active-color;
    background-color: $active-background;
    @if $enable-gradients {
      background-image: none; // Remove the gradient for the pressed/active state
    }
    border-color: $active-border;

    &:focus {
      // Avoid using mixin so we can pass custom focus shadow properly
      @if $enable-shadows and $btn-active-box-shadow != none {
        box-shadow: $btn-active-box-shadow, 0 0 0 $btn-focus-width rgba(mix($color, $border, 15%), .5);
      } @else {
        box-shadow: 0 0 0 $btn-focus-width rgba(mix($color, $border, 15%), .5);
      }
    }
  }
}

@mixin button-outline-variant(
  $color,
  $color-hover: color-yiq($color),
  $active-background: $color,
  $active-border: $color,
  $active-color: color-yiq($active-background)
) {
  color: $color;
  border-color: $color;

  &:hover {
    color: $color-hover;
    background-color: $active-background;
    border-color: $active-border;
  }

  &:focus,
  &.focus {
    box-shadow: 0 0 0 $btn-focus-width rgba($color, .5);
  }

  &.disabled,
  &:disabled {
    color: $color;
    background-color: transparent;
  }

  &:not(:disabled):not(.disabled):active,
  &:not(:disabled):not(.disabled).active,
  .show > &.dropdown-toggle {
    color: $active-color;
    background-color: $active-background;
    border-color: $active-border;

    &:focus {
      // Avoid using mixin so we can pass custom focus shadow properly
      @if $enable-shadows and $btn-active-box-shadow != none {
        box-shadow: $btn-active-box-shadow, 0 0 0 $btn-focus-width rgba($color, .5);
      } @else {
        box-shadow: 0 0 0 $btn-focus-width rgba($color, .5);
      }
    }
  }
}

// Button sizes
@mixin button-size($padding-y, $padding-x, $font-size, $border-radius) {
  padding: $padding-y $padding-x;
  @include font-size($font-size);
  // Manually declare to provide an override to the browser default
  @include border-radius($border-radius, 0);
}