--- layout: docs title: Theming Bootstrap description: Customize Bootstrap 4 with our new built-in Sass variables for global style preferences for easy theming and component changes. group: getting-started toc: true --- ## 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: ```text 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. ```text 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. ```scss // Custom.scss // Option A: Include all of Bootstrap @import "../node_modules/bootstrap/scss/bootstrap"; ``` ```scss // 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 `
` when importing and compiling Bootstrap via npm: ```scss // Your variable overrides $body-bg: #000; $body-color: #111; // Bootstrap and its default variables @import "../node_modules/bootstrap/scss/bootstrap"; ``` Repeat as necessary for any variable in Bootstrap, including the global options below. ### Maps and loops Bootstrap 4 includes a handful of Sass maps, key value pairs that make it easier to generate families of related CSS. We use Sass maps for our colors, grid breakpoints, and more. Just like Sass variables, all Sass maps include the `!default` flag and can be overridden and extended. Some of our Sass maps are merged into empty ones by default. This is done to allow easy expansion of a given Sass map, but comes at the cost of making _removing_ items from a map slightly more difficult. #### Modify map To modify an existing color in our `$theme-colors` map, add the following to your custom Sass file: ```scss $theme-colors: ( "primary": #0074d9, "danger": #ff4136 ); ``` #### Add to map To add a new color to `$theme-colors`, add the new key and value: ```scss $theme-colors: ( "custom-color": #900 ); ``` #### Remove from map To remove colors from `$theme-colors`, or any other map, use `map-remove`. Be aware you must insert it between our requirements and options: ```scss // 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 `$theme-colors` for links, buttons, and form states. Replacing the values of these keys should present no issues, but removing them may cause Sass compilation issues. In these instances, you'll need to modify the Sass code that makes use of those values. ### Functions Bootstrap utilizes several Sass functions, but only a subset are applicable to general theming. We've included three functions for getting values from the color maps: ```scss @function color($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, $key); } ``` These allow you to pick one color from a Sass map much like how you'd use a color variable from v3. ```scss .custom-element { color: gray("100"); background-color: theme-color("dark"); } ``` We also have another function for getting a particular _level_ of color from the `$theme-colors` map. Negative level values will lighten the color, while higher levels will darken. ```scss @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. ```scss .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](https://en.wikipedia.org/wiki/YIQ) 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: ```scss @each $color, $value in $theme-colors { .swatch-#{$color} { color: color-yiq($value); } } ``` It can also be used for one-off contrast needs: ```scss .custom-element { color: color-yiq(#000); // returns `color: #fff` } ``` You can also specify a base color with our color map functions: ```scss .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: ```scss $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: ```scss $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 | | -------------------------------------------- | ---------------------------------- | -------------------------------------------------------------------------------------- | | `$spacer` | `1rem` (default), or any value > 0 | Specifies the default spacer value to programmatically generate our [spacer utilities]({{< docsref "/utilities/spacing" >}}). | | `$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]({{< docsref "/getting-started/accessibility#reduced-motion" >}}), 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]({{< docsref "/content/typography#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` 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 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](#grays) we already include.