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Accessibility tip: Using color to add meaning only provides a visual indication, which will not be conveyed to users of assistive technologies like screen readers. Please ensure the meaning is obvious from the content itself (e.g., the visible text with a sufficient color contrast ) or is included through alternative means, such as additional text hidden with the


   .visually-hidden

class.

¥Background color

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。后台实用程序不会设置 color ，因此在某些情况下你需要使用 .text-* 颜色实用程序。

¥Similar to the contextual text color classes, set the background of an element to any contextual class. Background utilities do not set color , so in some cases you’ll want to use .text-* color utilities .

Background utilities like


   .bg-*

that generated from our original


   $theme-colors

Sass map don’t yet respond to color modes, however, any


   .bg-*-subtle

utility will. This will be resolved in v6.

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

¥Background gradient

通过添加 .bg-gradient 类，将线性渐变作为背景图片添加到背景中。该渐变从半透明的白色开始，逐渐淡出到底部。

¥By adding a .bg-gradient class, a linear gradient is added as background image to the backgrounds. This gradient starts with a semi-transparent white which fades out to the bottom.

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

¥Do you need a gradient in your custom CSS? Just add 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

.bg-black.bg-gradient

¥Opacity

Added in v5.1.0

从 v5.1.0 开始， background-color 工具是使用 CSS 变量通过 Sass 生成的。这允许实时颜色变化而无需编译和动态 Alpha 透明度变化。

¥As of v5.1.0, background-color utilities are generated with Sass using CSS variables. This allows for real-time color changes without compilation and dynamic alpha transparency changes.

怎么运行的

¥How it works

考虑我们默认的 .bg-success 工具。

¥Consider our default .bg-success utility.

.bg-success {
  --bs-bg-opacity: 1;
  background-color: rgba(var(--bs-success-rgb), var(--bs-bg-opacity)) !important;

我们使用 --bs-success 的 RGB 版本（值为 25, 135, 84 ）CSS 变量，并附加第二个 CSS 变量 --bs-bg-opacity ，用于 alpha 透明度（由于本地 CSS 变量，默认值为 1 ）。这意味着你现在使用 .bg-success 时，计算出的 color 值为 rgba(25, 135, 84, 1) 。每个 .bg-* 类中的本地 CSS 变量避免了继承问题，因此工具的嵌套实例不会自动具有修改后的 alpha 透明度。

¥We use an RGB version of our --bs-success (with the value of 25, 135, 84 ) CSS variable and attached a second CSS variable, --bs-bg-opacity , for the alpha transparency (with a default value 1 thanks to a local CSS variable). That means anytime you use .bg-success now, your computed color value is rgba(25, 135, 84, 1) . The local CSS variable inside each .bg-* class avoids inheritance issues so nested instances of the utilities don’t automatically have a modified alpha transparency.

¥Example

要更改该不透明度，请通过自定义样式或内联样式覆盖 --bs-bg-opacity 。

¥To change that opacity, override --bs-bg-opacity via custom styles or inline styles.

This is default success background

This is 50% opacity success background

<div class="bg-success p-2 text-white">This is default success background</div>
<div class="bg-success p-2" style="--bs-bg-opacity: .5;">This is 50% opacity success background</div>

或者，从任何 .bg-opacity 工具中进行选择：

¥Or, choose from any of the .bg-opacity utilities:

This is default success background

This is 75% opacity success background

This is 50% opacity success background

This is 25% opacity success background

This is 10% opacity success background

<div class="bg-success p-2 text-white">This is default success background</div>
<div class="bg-success p-2 text-white bg-opacity-75">This is 75% opacity success background</div>
<div class="bg-success p-2 text-dark bg-opacity-50">This is 50% opacity success background</div>
<div class="bg-success p-2 text-dark bg-opacity-25">This is 25% opacity success background</div>
<div class="bg-success p-2 text-dark bg-opacity-10">This is 10% opacity success background</div>

除了以下 Sass 功能之外，请考虑阅读我们包含的颜色等的 CSS 自定义属性（又名 CSS 变量）。

¥In addition to the following Sass functionality, consider reading about our included CSS custom properties (aka CSS variables) for colors and more.

Sass 变量

¥Sass variables

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

¥Most background-color utilities are generated by our theme colors, reassigned from our generic color palette variables.

scss/_variables.scss

$gradient: linear-gradient(180deg, rgba($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工具。
¥Grayscale colors are also available, but only a subset are used to generate any utilities.
      scss/_variables.scss
用于在浅色和夜间模式下在 .bg-*-subtle 工具中设置 background-color 的变量：
¥Variables for setting background-color in .bg-*-subtle utilities in light and dark mode:
      scss/_variables.scss

$primary-bg-subtle:       tint-color($primary, 80%);
$secondary-bg-subtle:     tint-color($secondary, 80%);
$success-bg-subtle:       tint-color($success, 80%);
$info-bg-subtle:          tint-color($info, 80%);
$warning-bg-subtle:       tint-color($warning, 80%);
$danger-bg-subtle:        tint-color($danger, 80%);
$light-bg-subtle:         mix($gray-100, $white);
$dark-bg-subtle:          $gray-400;
      scss/_variables-dark.scss

$primary-bg-subtle-dark:            shade-color($primary, 80%);
$secondary-bg-subtle-dark:          shade-color($secondary, 80%);
$success-bg-subtle-dark:            shade-color($success, 80%);
$info-bg-subtle-dark:               shade-color($info, 80%);
$warning-bg-subtle-dark:            shade-color($warning, 80%);
$danger-bg-subtle-dark:             shade-color($danger, 80%);
$light-bg-subtle-dark:              $gray-800;
$dark-bg-subtle-dark:               mix($gray-800, $black);
Sass 映射 

¥Sass maps
然后主题颜色被放入 Sass 映射中，以便我们可以循环它们以生成我们的工具、组件修饰符等。
¥Theme colors are then put into a Sass map so we can loop over them to generate our utilities, component modifiers, and more.
      scss/_variables.scss
灰度颜色也可用作 Sass 贴图。该映射不用于生成任何工具。
¥Grayscale colors are also available as a Sass map. This map is not used to generate any utilities.
      scss/_variables.scss

$theme-colors-rgb: map-loop($theme-colors, to-rgb, "$value");
背景颜色不透明度建立在其自己的映射之上，由工具 API 使用：
¥Background color opacities build on that with their own map that’s consumed by the utilities API:
      scss/_maps.scss
    "black": to-rgb($black),
    "white": to-rgb($white),
    "body": to-rgb($body-bg)
$utilities-bg-colors: map-loop($utilities-bg, rgba-css-var, "$key", "bg");
$utilities-bg-subtle: (
  "primary-subtle": var(--#{$prefix}primary-bg-subtle),
  "secondary-subtle": var(--#{$prefix}secondary-bg-subtle),
  "success-subtle": var(--#{$prefix}success-bg-subtle),
  "info-subtle": var(--#{$prefix}info-bg-subtle),
  "warning-subtle": var(--#{$prefix}warning-bg-subtle),
  "danger-subtle": var(--#{$prefix}danger-bg-subtle),
  "light-subtle": var(--#{$prefix}light-bg-subtle),
  "dark-subtle": var(--#{$prefix}dark-bg-subtle)
颜色模式背景颜色也可用作 Sass 映射：
¥Color mode background colors are also available as a Sass map:
      scss/_maps.scss

$theme-colors-bg-subtle: (
  "primary": $primary-bg-subtle,
  "secondary": $secondary-bg-subtle,
  "success": $success-bg-subtle,
  "info": $info-bg-subtle,
  "warning": $warning-bg-subtle,
  "danger": $danger-bg-subtle,
  "light": $light-bg-subtle,
  "dark": $dark-bg-subtle,
      scss/_maps.scss

$theme-colors-bg-subtle-dark: (
  "primary": $primary-bg-subtle-dark,
  "secondary": $secondary-bg-subtle-dark,
  "success": $success-bg-subtle-dark,
  "info": $info-bg-subtle-dark,
  "warning": $warning-bg-subtle-dark,
  "danger": $danger-bg-subtle-dark,
  "light": $light-bg-subtle-dark,
  "dark": $dark-bg-subtle-dark,
Sass 混入 

¥Sass mixins
没有使用 mixins 来生成我们的背景工具，但我们确实有一些额外的 mixins 用于你想要创建自己的渐变的其他情况。
¥No mixins are used to generate our background utilities, but we do have some additional mixins for other situations where you’d like to create your own gradients.
      scss/mixins/_gradients.scss

// 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);
Sass 工具 API 

¥Sass utilities API
后台工具在 scss/_utilities.scss 中的工具 API 中声明。Learn how to use the utilities API.
¥Background utilities are declared in our utilities API in scss/_utilities.scss. Learn how to use the utilities API.
      scss/_utilities.scss
      "transparent": transparent,
      "body-secondary": rgba(var(--#{$prefix}secondary-bg-rgb), var(--#{$prefix}bg-opacity)),
      "body-tertiary": rgba(var(--#{$prefix}tertiary-bg-rgb), var(--#{$prefix}bg-opacity)),
"bg-opacity": (
  css-var: true,
  class: bg-opacity,
  values: (
    10: .1,
    25: .25,
    50: .5,
    75: .75,
    100: 1
"subtle-background-color": (
  property: background-color,
  class: bg,
  values: $utilities-bg-subtle