SOURCE

<button type="button" class="btn btn-primary disabled">Primary</button>
<button type="button" class="btn btn-secondary">Secondary</button>
<button type="button" class="btn btn-success">Success</button>
<button type="button" class="btn btn-danger">Danger</button>
<button type="button" class="btn btn-warning">Warning</button>
<button type="button" class="btn btn-info">Info</button>
<button type="button" class="btn btn-light">Light</button>
<button type="button" class="btn btn-dark">Dark</button>

<button type="button" class="btn btn-link">Link</button>
<div style="margin-bottom: 1em;"></div>
<button type="button" class="btn btn-outline-primary">Primary</button>
<button type="button" class="btn btn-outline-secondary" disabled>Secondary</button>
<button type="button" class="btn btn-outline-success">Success</button>
<button type="button" class="btn btn-outline-danger">Danger</button>
<button type="button" class="btn btn-outline-warning">Warning</button>
<button type="button" class="btn btn-outline-info">Info</button>
<button type="button" class="btn btn-outline-light">Light</button>
<button type="button" class="btn btn-outline-dark">Dark</button>

// Variables
//
// Variables should follow the `$component-state-property-size` formula for
// consistent naming. Ex: $nav-link-disabled-color and $modal-content-box-shadow-xs.


//
// Color system
//

// stylelint-disable
$white:    #fff !default;
$gray-100: #f8f9fa !default;
$gray-200: #e9ecef !default;
$gray-300: #dee2e6 !default;
$gray-400: #ced4da !default;
$gray-500: #adb5bd !default;
$gray-600: #6c757d !default;
$gray-700: #495057 !default;
$gray-800: #343a40 !default;
$gray-900: #212529 !default;
$black:    #000 !default;

$grays: () !default;
$grays: map-merge((
  "100": $gray-100,
  "200": $gray-200,
  "300": $gray-300,
  "400": $gray-400,
  "500": $gray-500,
  "600": $gray-600,
  "700": $gray-700,
  "800": $gray-800,
  "900": $gray-900
), $grays);

$blue:    #007bff !default;
$indigo:  #6610f2 !default;
$purple:  #6f42c1 !default;
$pink:    #e83e8c !default;
$red:     #dc3545 !default;
$orange:  #fd7e14 !default;
$yellow:  #ffc107 !default;
$green:   #28a745 !default;
$teal:    #20c997 !default;
$cyan:    #17a2b8 !default;

$colors: () !default;
$colors: map-merge((
  "blue":       $blue,
  "indigo":     $indigo,
  "purple":     $purple,
  "pink":       $pink,
  "red":        $red,
  "orange":     $orange,
  "yellow":     $yellow,
  "green":      $green,
  "teal":       $teal,
  "cyan":       $cyan,
  "white":      $white,
  "gray":       $gray-600,
  "gray-dark":  $gray-800
), $colors);

$primary:       $blue !default;
$secondary:     $gray-600 !default;
$success:       $green !default;
$info:          $cyan !default;
$warning:       $yellow !default;
$danger:        $red !default;
$light:         $gray-100 !default;
$dark:          $gray-800 !default;

$theme-colors: () !default;
$theme-colors: map-merge((
  "primary":    $primary,
  "secondary":  $secondary,
  "success":    $success,
  "info":       $info,
  "warning":    $warning,
  "danger":     $danger,
  "light":      $light,
  "dark":       $dark
), $theme-colors);

$input-btn-padding-x: .75rem;
$input-btn-padding-y: .375rem;
$input-btn-padding-x-sm: .5rem;
$input-btn-padding-y-sm: .25rem;
$input-btn-padding-y-lg: .5rem;
$input-btn-padding-x-lg: 1rem;

$yiq-contrasted-threshold: 150 !default;

// Customize the light and dark text colors for use in our YIQ color contrast function.
$yiq-text-dark: $gray-900 !default;
$yiq-text-light: $white !default;
@function color-yiq($color) {
  $r: red($color);
  $g: green($color);
  $b: blue($color);

  $yiq: (($r * 299) + ($g * 587) + ($b * 114)) / 1000;

  @if ($yiq >= $yiq-contrasted-threshold) {
    @return $yiq-text-dark;
  } @else {
    @return $yiq-text-light;
  }
}

$btn-focus-box-shadow: 0 0 0 .25rem rgba($primary, .125);
$btn-active-box-shadow: inset 0 3px 3px 0 rgba(#000, .125);
$btn-box-shadow: inset 0 1px 0 rgba(#fff, .15), 0 1px 1px rgba(#000, .075);
.btn{
  display: inline-block;
  line-height: 1.5;
  text-align: center;
  vertical-align: middle;
  white-space: nowrap;
  padding: $input-btn-padding-y $input-btn-padding-x;
  border: 1px solid transparent;
  border-radius: .25rem;
  outline: 0;
  &.disabled, &:disabled{
    opacity: .65;
    box-shadow: none; // 干掉 focus 效果
  }
  &:not(.disabled):not(:disabled){
    &.focus, &:focus{
      box-shadow: $btn-focus-box-shadow;
      outline: 0; // 把 webkit 的默认样式干掉
    }
    cursor: pointer;
    &.active,&:active{
      // 点击效果
      box-shadow: $btn-active-box-shadow;
      &.focus, &:focus{
        box-shadow: $btn-focus-box-shadow, $btn-active-box-shadow;
      }
    }
  }
} 

@each $color-name, $color-value in $theme-colors{
  .btn-#{$color-name}{
    color: color-yiq($color-value);
    background: $color-value;
    &:not(.disabled):not(:disabled){
      &:hover{
        $hover-color: darken($color-value, 10%);
        border-color: $hover-color;
        background-color: $hover-color;
        color: color-yiq($hover-color);
      }
      &.focus, &:focus{
        box-shadow: 0 0 0 .25rem rgba($color-value, .5);
      }
    }
  }
  .btn-outline-#{$color-name}{
    color: $color-value;
    background: transparent;
    border: 1px solid $color-value;
    &:not(:disabled):not(.disabled){
      &:focus, &.focus,&.hover, &:hover{
        background: $color-value;
        color: color-yiq($color-value);
      }
      &:focus, &.focus{
        box-shadow: 0 0 0 .25rem rgba($color-value, .5);
      }
    }
  }
  
}