背景（Background）

通过背景色传达意义，并添加渐变装饰。

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背景色

与上下文文本颜色类类似，将元素的背景设置为任何上下文类。背景实用程序不设置颜色，因此在某些情况下您需要使用.text-*颜色实用程序。

.bg-primary
.bg-secondary
.bg-success
.bg-danger
.bg-warning
.bg-info
.bg-light
.bg-dark
.bg-body
.bg-white
.bg-transparent
<div class="p-3 mb-2 bg-primary text-white">.bg-primary</div>
        <div class="p-3 mb-2 bg-secondary text-white">.bg-secondary</div>
        <div class="p-3 mb-2 bg-success text-white">.bg-success</div>
        <div class="p-3 mb-2 bg-danger text-white">.bg-danger</div>
        <div class="p-3 mb-2 bg-warning text-dark">.bg-warning</div>
        <div class="p-3 mb-2 bg-info text-dark">.bg-info</div>
        <div class="p-3 mb-2 bg-light text-dark">.bg-light</div>
        <div class="p-3 mb-2 bg-dark text-white">.bg-dark</div>
        <div class="p-3 mb-2 bg-body text-dark">.bg-body</div>
        <div class="p-3 mb-2 bg-white text-dark">.bg-white</div>
        <div class="p-3 mb-2 bg-transparent text-dark">.bg-transparent</div>

背景梯度

通过添加一个.bg-gradient梯度类，将一个线性梯度作为背景图像添加到背景中。这个渐变从一个半透明的白色开始，逐渐消失到底部。

你的自定义CSS需要渐变吗？只需添加背景图像background-image: var(--bs-gradient);。

.bg-primary.bg-gradient
.bg-secondary.bg-gradient
.bg-success.bg-gradient
.bg-danger.bg-gradient
.bg-warning.bg-gradient
.bg-info.bg-gradient
.bg-light.bg-gradient
.bg-dark.bg-gradient

Sass

除了下面的Sass功能外，请考虑阅读我们的CSS自定义属性（aka CSS variables）中包含的颜色等内容。

Variables

大多数background-color工具是由我们的主题颜色生成的，从我们的通用调色板变量重新分配。

$blue:    #0d6efd;
        $indigo:  #6610f2;
        $purple:  #6f42c1;
        $pink:    #d63384;
        $red:     #dc3545;
        $orange:  #fd7e14;
        $yellow:  #ffc107;
        $green:   #198754;
        $teal:    #20c997;
        $cyan:    #0dcaf0;
        
$primary:       $blue;
        $secondary:     $gray-600;
        $success:       $green;
        $info:          $cyan;
        $warning:       $yellow;
        $danger:        $red;
        $light:         $gray-100;
        $dark:          $gray-900;
        
$gradient: linear-gradient(180deg, rgba($white, .15), rgba($white, 0));

灰度颜色也可用，但只有一个子集用于生成任何实用程序。 

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

Map

然后将主题颜色放入Sass映射中，这样我们就可以循环使用它们来生成实用程序、组件修改器等等。

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

灰度颜色也可用作Sass地图。 此映射不用于生成任何实用程序。 

$grays: (
        "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
        );

Mixins

没有使用mixin来生成我们的后台实用程序，但是我们有一些额外的mixin，用于您希望创建自己的渐变的其他情况。

@mixin gradient-bg($color: null) {
        background-color: $color;
        
        @if $enable-gradients {
        background-image: var(--#{$variable-prefix}gradient);
        }
        }
        
// Horizontal gradient, from left to right
        //
        // Creates two color stops, start and end, by specifying a color and position for each color stop.
        @mixin gradient-x($start-color: $gray-700, $end-color: $gray-800, $start-percent: 0%, $end-percent: 100%) {
        background-image: linear-gradient(to right, $start-color $start-percent, $end-color $end-percent);
        }
        
        // Vertical gradient, from top to bottom
        //
        // Creates two color stops, start and end, by specifying a color and position for each color stop.
        @mixin gradient-y($start-color: $gray-700, $end-color: $gray-800, $start-percent: null, $end-percent: null) {
        background-image: linear-gradient(to bottom, $start-color $start-percent, $end-color $end-percent);
        }
        
        @mixin gradient-directional($start-color: $gray-700, $end-color: $gray-800, $deg: 45deg) {
        background-image: linear-gradient($deg, $start-color, $end-color);
        }
        
        @mixin gradient-x-three-colors($start-color: $blue, $mid-color: $purple, $color-stop: 50%, $end-color: $red) {
        background-image: linear-gradient(to right, $start-color, $mid-color $color-stop, $end-color);
        }
        
        @mixin gradient-y-three-colors($start-color: $blue, $mid-color: $purple, $color-stop: 50%, $end-color: $red) {
        background-image: linear-gradient($start-color, $mid-color $color-stop, $end-color);
        }
        
        @mixin gradient-radial($inner-color: $gray-700, $outer-color: $gray-800) {
        background-image: radial-gradient(circle, $inner-color, $outer-color);
        }
        
        @mixin gradient-striped($color: rgba($white, .15), $angle: 45deg) {
        background-image: linear-gradient($angle, $color 25%, transparent 25%, transparent 50%, $color 50%, $color 75%, transparent 75%, transparent);
        }

Utilities API

后台实用程序在scss的实用程序API中声明scss/_utilities.scss, href="https://www.bootstrap.cn/doc/read/162.html#using-the-api">了解如何使用实用程序API。

    "background-color": (
        property: background-color,
        class: bg,
        values: map-merge(
          $theme-colors,
          (
            "body": $body-bg,
            "white": $white,
            "transparent": transparent
          )
        )
        ),
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