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Theming Bootstrap

Customize Bootstrap 4 with our new built-in Sass variables for global style preferences for easy theming and component changes.

Introduction

In Bootstrap 3, theming was largely driven by variable overrides in LESS, custom CSS, and a separate theme stylesheet that we included in our dist files. With some effort, one could completely redesign the look of Bootstrap 3 without touching the core files. Bootstrap 4 provides a familiar, but slightly different approach.

Now, theming is accomplished by Sass variables, Sass maps, and custom CSS. There's no more dedicated theme stylesheet; instead, you can enable the built-in theme to add gradients, shadows, and more.

Sass

Utilize our source Sass files to take advantage of variables, maps, mixins, and more. In our build we've increased the Sass rounding precision to 6 (by default it's 5) to prevent issues with browser rounding.

File structure

Whenever possible, avoid modifying Bootstrap's core files. For Sass, that means creating your own stylesheet that imports Bootstrap so you can modify and extend it. Assuming you're using a package manager like npm, you'll have a file structure that looks like this:

your-project/
├── scss
│   └── custom.scss
└── node_modules/
    └── bootstrap
        ├── js
        └── scss

If you've downloaded our source files and aren't using a package manager, you'll want to manually setup something similar to that structure, keeping Bootstrap's source files separate from your own.

your-project/
├── scss
│   └── custom.scss
└── bootstrap/
    ├── js
    └── scss

Importing

In your custom.scss, you'll import Bootstrap's source Sass files. You have two options: include all of Bootstrap, or pick the parts you need. We encourage the latter, though be aware there are some requirements and dependencies across our components. You also will need to include some JavaScript for our plugins.

// Custom.scss
// Option A: Include all of Bootstrap

@import "../node_modules/bootstrap/scss/bootstrap";
// Custom.scss
// Option B: Include parts of Bootstrap

// Required
@import "../node_modules/bootstrap/scss/functions";
@import "../node_modules/bootstrap/scss/variables";
@import "../node_modules/bootstrap/scss/mixins";

// Optional
@import "../node_modules/bootstrap/scss/reboot";
@import "../node_modules/bootstrap/scss/type";
@import "../node_modules/bootstrap/scss/images";
@import "../node_modules/bootstrap/scss/code";
@import "../node_modules/bootstrap/scss/grid";

With that setup in place, you can begin to modify any of the Sass variables and maps in your custom.scss. You can also start to add parts of Bootstrap under the // Optional section as needed. We suggest using the full import stack from our bootstrap.scss file as your starting point.

Variable defaults

Every Sass variable in Bootstrap 4 includes the !default flag allowing you to override the variable's default value in your own Sass without modifying Bootstrap's source code. Copy and paste variables as needed, modify their values, and remove the !default flag. If a variable has already been assigned, then it won't be re-assigned by the default values in Bootstrap.

You will find the complete list of Bootstrap's variables in scss/_variables.scss. Some variables are set to null, these variables don't output the property unless they are overridden in your configuration.

Variable overrides within the same Sass file can come before or after the default variables. However, when overriding across Sass files, your overrides must come before you import Bootstrap's Sass files.

Here's an example that changes the background-color and color for the <body> when importing and compiling Bootstrap via npm:

// Your variable overrides
$body-bg: #000;
$body-color: #111;

// Bootstrap and its default variables
@import "../node_modules/bootstrap/scss/bootstrap";

Repeat as necessary for any variable in Bootstrap, including the global options below.

Maps and loops

Bootstrap 4 includes a handful of Sass maps, key value pairs that make it easier to generate families of related CSS. We use Sass maps for our colors, grid breakpoints, and more. Just like Sass variables, all Sass maps include the !default flag and can be overridden and extended.

Some of our Sass maps are merged into empty ones by default. This is done to allow easy expansion of a given Sass map, but comes at the cost of making removing items from a map slightly more difficult.

Modify map

All variables in the $theme-colors map are defined as standalone variables. To modify an existing color in our $theme-colors map, add the following to your custom Sass file:

$primary: #0074d9;
$danger: #ff4136;

Later on, theses variables are set in Bootstrap's $theme-colors map:

$theme-colors: (
  "primary": $primary,
  "danger": $danger
);

Add to map

To add a new color to $theme-colors, add the new key and value:

$theme-colors: (
  "custom-color": #900
);

Remove from map

To remove colors from $theme-colors, or any other map, use map-remove. Be aware you must insert it between our requirements and options:

// Required
@import "../node_modules/bootstrap/scss/functions";
@import "../node_modules/bootstrap/scss/variables";
@import "../node_modules/bootstrap/scss/mixins";

$theme-colors: map-remove($theme-colors, "info", "light", "dark");

// Optional
@import "../node_modules/bootstrap/scss/root";
@import "../node_modules/bootstrap/scss/reboot";
@import "../node_modules/bootstrap/scss/type";
...

Required keys

Bootstrap assumes the presence of some specific keys within Sass maps as we used and extend these ourselves. As you customize the included maps, you may encounter errors where a specific Sass map's key is being used.

For example, we use the primary, success, and danger keys from $theme-colors for links, buttons, and form states. Replacing the values of these keys should present no issues, but removing them may cause Sass compilation issues. In these instances, you'll need to modify the Sass code that makes use of those values.

Functions

In Bootstrap 5, we've dropped the color(), theme-color() and gray() functions because the values are also available as standalone variables. So instead of using theme-color("primary"), you can now just use the $primary variable.

.custom-element {
  color: $gray-100;
  background-color: $dark;
}

We also have a function for getting a particular level of color. Negative level values will lighten the color, while higher levels will darken.

@function color-level($color: $primary, $level: 0) {
  $color-base: if($level > 0, #000, #fff);
  $level: abs($level);

  @return mix($color-base, $color, $level * $theme-color-interval);
}

In practice, you'd call the function and pass in two parameters: the name of the color from $theme-colors (e.g., primary or danger) and a numeric level.

.custom-element {
  color: color-level($primary, -10);
}

Color contrast

An additional function we include in Bootstrap is the color contrast function, color-yiq. It utilizes the YIQ color space to automatically return a light (#fff) or dark (#111) contrast color based on the specified base color. This function is especially useful for mixins or loops where you're generating multiple classes.

For example, to generate color swatches from our $theme-colors map:

@each $color, $value in $theme-colors {
  .swatch-#{$color} {
    color: color-yiq($value);
  }
}

It can also be used for one-off contrast needs:

.custom-element {
  color: color-yiq(#000); // returns `color: #fff`
}

You can also specify a base color with our color map functions:

.custom-element {
  color: color-yiq($dark); // returns `color: #fff`
}

Escape SVG

We use the escape-svg function to escape the <, > and # characters for SVG background images. These characters need to be escaped to properly render the background images in IE.

Add and Subtract functions

We use the add and subtract functions to wrap the CSS calc function. The primary purpose of these functions is to avoid errors when a “unitless” 0 value is passed into a calc expression. Expressions like calc(10px - 0) will return an error in all browsers, despite being mathematically correct.

Example where the calc is valid:

$border-radius: .25rem;
$border-width: 1px;

.element {
  // Output calc(.25rem - 1px) is valid
  border-radius: calc($border-radius - $border-width);
}

.element {
  // Output the same calc(.25rem - 1px) as above
  border-radius: subtract($border-radius, $border-width);
}

Example where the calc is invalid:

$border-radius: .25rem;
$border-width: 0;

.element {
  // Output calc(.25rem - 0) is invalid
  border-radius: calc($border-radius - $border-width);
}

.element {
  // Output .25rem
  border-radius: subtract($border-radius, $border-width);
}

Sass options

Customize Bootstrap 4 with our built-in custom variables file and easily toggle global CSS preferences with new $enable-* Sass variables. Override a variable's value and recompile with npm run test as needed.

You can find and customize these variables for key global options in Bootstrap's scss/_variables.scss file.

Variable Values Description
$spacer 1rem (default), or any value > 0 Specifies the default spacer value to programmatically generate our spacer utilities.
$enable-rounded true (default) or false Enables predefined border-radius styles on various components.
$enable-shadows true or false (default) Enables predefined box-shadow styles on various components.
$enable-gradients true or false (default) Enables predefined gradients via background-image styles on various components.
$enable-transitions true (default) or false Enables predefined transitions on various components.
$enable-prefers-reduced-motion-media-query true (default) or false Enables the prefers-reduced-motion media query, which suppresses certain animations/transitions based on the users’ browser/operating system preferences.
$enable-grid-classes true (default) or false Enables the generation of CSS classes for the grid system (e.g., .container, .row, .col-md-1, etc.).
$enable-caret true (default) or false Enables pseudo element caret on .dropdown-toggle.
$enable-pointer-cursor-for-buttons true (default) or false Add “hand” cursor to non-disabled button elements.
$enable-rfs true (default) or false Globally enables RFS.
$enable-validation-icons true (default) or false Enables background-image icons within textual inputs and some custom forms for validation states.
$enable-deprecation-messages true or false (default) Set to true to show warnings when using any of the deprecated mixins and functions that are planned to be removed in v5.

Color

Many of Bootstrap's various components and utilities are built through a series of colors defined in a Sass map. This map can be looped over in Sass to quickly generate a series of rulesets.

All colors

All colors available in Bootstrap 5, are available as Sass variables and a Sass map in scss/_variables.scss file. To avoid increased file sizes, we do not create classes for each of these variables.

Sass cannot programmatically generate variables, so we must manually create them ourselves. We specify the midpoint value (500) and use custom color functions to tint (lighten) or shade (darken) our colors via Sass's mix() color function. Using mix() is not the same as lighten() and darken()—the former blends the specified color with white or black, while the latter only adjusts the lightness value of each color. The result is a much more complete suite of colors, as shown in this CodePen demo.

Our tint-color() and shade-color() functions use mix() alongside our $theme-color-interval variable, which specifies a stepped percentage value for each mixed color we need. See the scss/_functions.scss and scss/_variables.scss files for the full source code.

Here's how you can use these in your Sass:

.alpha { color: $purple; }
.beta {
  color: $yellow-300;
  background-color: $indigo-900;
}

Color utility classes are also available for setting color and background-color.

In the future, we'll aim to provide Sass maps and variables for shades of each color as we've done with the grayscale colors below.

Theme colors

We use a subset of all colors to create a smaller color palette for generating color schemes, also available as Sass variables and a Sass map in Bootstrap's scss/_variables.scss file.

Primary
Secondary
Success
Danger
Warning
Info
Light
Dark

Grays

An expansive set of gray variables and a Sass map in scss/_variables.scss for consistent shades of gray across your project. Note that these are “cool grays”, which tend towards a subtle blue tone, rather than neutral grays.

100
200
300
400
500
600
700
800
900

Within scss/_variables.scss, you'll find Bootstrap's color variables and Sass map. Here's an example of the $colors Sass map:

$colors: (
  "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
) !default;

Add, remove, or modify values within the map to update how they're used in many other components. Unfortunately at this time, not every component utilizes this Sass map. Future updates will strive to improve upon this. Until then, plan on making use of the ${color} variables and this Sass map.

Components

Many of Bootstrap's components and utilities are built with @each loops that iterate over a Sass map. This is especially helpful for generating variants of a component by our $theme-colors and creating responsive variants for each breakpoint. As you customize these Sass maps and recompile, you'll automatically see your changes reflected in these loops.

Modifiers

Many of Bootstrap's components are built with a base-modifier class approach. This means the bulk of the styling is contained to a base class (e.g., .btn) while style variations are confined to modifier classes (e.g., .btn-danger). These modifier classes are built from the $theme-colors map to make customizing the number and name of our modifier classes.

Here are two examples of how we loop over the $theme-colors map to generate modifiers to the .alert component and all our .bg-* background utilities.

// Generate alert modifier classes
@each $color, $value in $theme-colors {
  .alert-#{$color} {
    @include alert-variant(color-level($color, -10), color-level($color, -9), color-level($color, 6));
  }
}

// Generate `.bg-*` color utilities
@each $color, $value in $theme-colors {
  @include bg-variant('.bg-#{$color}', $value);
}

Responsive

These Sass loops aren't limited to color maps, either. You can also generate responsive variations of your components or utilities. Take for example our responsive text alignment utilities where we mix an @each loop for the $grid-breakpoints Sass map with a media query include.

@each $breakpoint in map-keys($grid-breakpoints) {
  @include media-breakpoint-up($breakpoint) {
    $infix: breakpoint-infix($breakpoint, $grid-breakpoints);

    .text#{$infix}-left   { text-align: left !important; }
    .text#{$infix}-right  { text-align: right !important; }
    .text#{$infix}-center { text-align: center !important; }
  }
}

Should you need to modify your $grid-breakpoints, your changes will apply to all the loops iterating over that map.

CSS variables

Bootstrap 4 includes around two dozen CSS custom properties (variables) in its compiled CSS. These provide easy access to commonly used values like our theme colors, breakpoints, and primary font stacks when working in your browser's Inspector, a code sandbox, or general prototyping.

Available variables

Here are the variables we include (note that the :root is required). They're located in our _root.scss file.

:root {
  --blue: #0d6efd;
  --indigo: #6610f2;
  --purple: #6f42c1;
  --pink: #d63384;
  --red: #dc3545;
  --orange: #fd7e14;
  --yellow: #ffc107;
  --green: #28a745;
  --teal: #20c997;
  --cyan: #17a2b8;
  --white: #fff;
  --gray: #6c757d;
  --gray-dark: #343a40;
  --primary: #0d6efd;
  --secondary: #6c757d;
  --success: #28a745;
  --info: #17a2b8;
  --warning: #ffc107;
  --danger: #dc3545;
  --light: #f8f9fa;
  --dark: #343a40;
  --font-family-sans-serif: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, "Noto Sans", sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol", "Noto Color Emoji";
  --font-family-monospace: SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace;
}

Examples

CSS variables offer similar flexibility to Sass's variables, but without the need for compilation before being served to the browser. For example, here we're resetting our page's font and link styles with CSS variables.

body {
  font: 1rem/1.5 var(--font-family-sans-serif);
}
a {
  color: var(--blue);
}