背景（Background）
通過背景色傳達意義，并添加漸變裝飾。
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背景色
與上下文文本顏色類類似，將元素的背景設置為任何上下文類。背景實用程序不設置顏色，因此在某些情況下您需要使用.text-*顏色實用程序。
.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
通過添加一個.bg-gradient梯度類，將一個線性梯度作為背景圖像添加到背景中。這個漸變從一個半透明的白色開始，逐漸消失到底部。
你的自定義CSS需要漸變嗎？只需添加背景圖像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，請考慮閱讀我們的CSS自定義屬性（aka CSS variables）中包含的顏色等內容。
Variables
大多數background-color工具是由我們的主題顏色生成的，從我們的通用調色板變量重新分配。
$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));
灰度顏色也可用，但只有一個子集用于生成任何實用程序。
$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
然后將主題顏色放入Sass映射中，這樣我們就可以循環使用它們來生成實用程序、組件修改器等等。
$theme-colors: (
"primary": $primary,
"secondary": $secondary,
"success": $success,
"info": $info,
"warning": $warning,
"danger": $danger,
"light": $light,
"dark": $dark
);
灰度顏色也可用作Sass地圖。 此映射不用于生成任何實用程序。
$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": $gray-900
);
Mixins
沒有使用mixin來生成我們的后臺實用程序，但是我們有一些額外的mixin，用于您希望創建自己的漸變的其他情況。
@mixin gradient-bg($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
后臺實用程序在scss的實用程序API中聲明scss/_utilities.scss,
href="http://www.roadlinkinfra.com/doc/read/162.html#using-the-api">了解如何使用實用程序API。
"background-color": (
property: background-color,
class: bg,
values: map-merge(
$theme-colors,
(
"body": $body-bg,
"white": $white,
"transparent": transparent
)
)
),