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 when compiling Sass using your own asset pipeline.
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 // Include any default variable overrides here (though functions won't be available) @import "../node_modules/bootstrap/scss/bootstrap"; // Then add additional custom code here // Custom.scss // Option B: Include parts of Bootstrap // 1. Include functions first (so you can manipulate colors, SVGs, calc, etc) @import "../node_modules/bootstrap/scss/functions"; // 2. Include any default variable overrides here // 3. Include remainder of required Bootstrap stylesheets @import "../node_modules/bootstrap/scss/variables"; @import "../node_modules/bootstrap/scss/mixins"; // 4. Include any optional Bootstrap components as you like @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 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 must come after our functions, variables, and mixins are imported, but before the rest of the imports.
Here’s an example that changes the background-color and color for the when importing and compiling Bootstrap via npm:
@import "../node_modules/bootstrap/scss/functions"; // Default variable overrides $body-bg: #000; $body-color: #111; // Required @import "../node_modules/bootstrap/scss/variables"; @import "../node_modules/bootstrap/scss/mixins"; // Optional Bootstrap components here @import "../node_modules/bootstrap/scss/root"; @import "../node_modules/bootstrap/scss/reboot"; @import "../node_modules/bootstrap/scss/type"; // etc Repeat as necessary for any variable in Bootstrap, including the global options below.
Get started with Bootstrap via npm with our starter project! Head to the twbs/bootstrap-npm-starter template repository to see how to build and customize Bootstrap in your own npm project. Includes Sass compiler, Autoprefixer, Stylelint, PurgeCSS, and Bootstrap Icons.
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
To modify an existing color in our $theme-colors map, add the following to your custom Sass file:
$theme-colors: ( "primary": #0074d9, "danger": #ff4136 ); 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 $theme-colors for links, buttons, and form states. Replacing the values of these keys should present no issues, but removing them may cause Sass compilation issues. In these instances, you’ll need to modify the Sass code that makes use of those values.
Functions
Bootstrap utilizes several Sass functions, but only a subset are applicable to general theming. We’ve included three functions for getting values from the color maps:
@function color($key: "blue") @return map-get($colors, $key); > @function theme-color($key: "primary") @return map-get($theme-colors, $key); > @function gray($key: "100") @return map-get($grays, $key); > These allow you to pick one color from a Sass map much like how you’d use a color variable from v3.
.custom-element color: gray("100"); background-color: theme-color("dark"); > We also have another function for getting a particular level of color from the $theme-colors map. Negative level values will lighten the color, while higher levels will darken.
@function theme-color-level($color-name: "primary", $level: 0) $color: theme-color($color-name); $color-base: if($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: theme-color-level(primary, -10); > Additional functions could be added in the future or your own custom Sass to create level functions for additional Sass maps, or even a generic one if you wanted to be more verbose.
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(theme-color("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. When using the escape-svg function, data URIs must be quoted.
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 decorative box-shadow styles on various components. Does not affect box-shadow s used for focus states. |
| $enable-gradients | true or false (default) | Enables predefined gradients via background-image styles on various components. |
| $enable-transitions | true (default) or false | Enables predefined transition s 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-hover-media-query | true or false (default) | Deprecated |
| $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-print-styles | true (default) or false | Enables styles for optimizing printing. |
| $enable-responsive-font-sizes | true or false (default) | Enables responsive font sizes. |
| $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 (default) or false | Set to false to hide 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 4, are available as Sass variables and a Sass map in scss/_variables.scss file. This will be expanded upon in subsequent minor releases to add additional shades, much like the grayscale palette we already include.