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.
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.
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:
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.
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.
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.
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:
Bootstrap 5 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.
From Bootstrap 5, we decided to [ditch the map merges](https://github.com/twbs/bootstrap/pull/28508) we had in `v4`. This way we have more control over removing redundant values.
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:
{{<highlightscss>}}
$primary: #0074d9;
$danger: #ff4136;
{{</highlight>}}
Later on, theses variables are set in Bootstrap's `$theme-colors` map:
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 `$theme-colors` for links, buttons, and form states. Replacing the values of these keys should present no issues, but removing them may cause Sass compilation issues. In these instances, you'll need to modify the Sass code that makes use of those values.
In Bootstrap 5, we've dropped the `color()`, `theme-color()` and `gray()` functions because the values are also available as standalone variables. So instead of using `theme-color("primary")`, you can now just use the `$primary` variable.
In practice, you'd call the function and pass in two parameters: the name of the color from `$theme-colors` (e.g., primary or danger) and a numeric level.
An additional function we include in Bootstrap is the color contrast function, `color-yiq`. It utilizes the [YIQ color space](https://en.wikipedia.org/wiki/YIQ) to automatically return a light (`#fff`) or dark (`#111`) contrast color based on the specified base color. This function is especially useful for mixins or loops where you're generating multiple classes.
We use the `add` and `subtract` functions to wrap the CSS `calc` function. The primary purpose of these functions is to avoid errors when a "unitless" `0` value is passed into a `calc` expression. Expressions like `calc(10px - 0)` will return an error in all browsers, despite being mathematically correct.
Customize Bootstrap 4 with our built-in custom variables file and easily toggle global CSS preferences with new `$enable-*` Sass variables. Override a variable's value and recompile with `npm run test` as needed.
| `$spacer` | `1rem` (default), or any value > 0 | Specifies the default spacer value to programmatically generate our [spacer utilities]({{< docsref "/utilities/spacing" >}}). |
| `$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. |
| `$enable-validation-icons` | `true` (default) or `false` | Enables `background-image` icons within textual inputs and some custom forms for validation states. |
| `$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`. |
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 available in Bootstrap 5 are available as Sass variables and as a Sass map in `scss/_variables.scss`. 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](https://codepen.io/emdeoh/pen/zYOQOPB).
Our `tint-color()` and `shade-color()` functions use `mix()` alongside our `$theme-color-interval` variable, which specifies a stepped percentage value for each mixed color we need. See the `scss/_functions.scss` and `scss/_variables.scss` files for the full source code.
We use a subset of all colors to create a smaller color palette for generating color schemes, also available as Sass variables and a Sass map in Bootstrap's `scss/_variables.scss` file.
An expansive set of gray variables and a Sass map in `scss/_variables.scss` for consistent shades of gray across your project. Note that these are "cool grays", which tend towards a subtle blue tone, rather than neutral grays.
Add, remove, or modify values within the map to update how they're used in many other components. Unfortunately at this time, not _every_ component utilizes this Sass map. Future updates will strive to improve upon this. Until then, plan on making use of the `${color}` variables and this Sass map.
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 `$theme-colors` map to generate modifiers to the `.alert` component and all our `.bg-*` background utilities.
These Sass loops aren't limited to color maps, either. You can also generate responsive variations of your components or utilities. Take for example our responsive text alignment utilities where we mix an `@each` loop for the `$grid-breakpoints` Sass map with a media query include.
Bootstrap 4 includes around two dozen [CSS custom properties (variables)](https://developer.mozilla.org/en-US/docs/Web/CSS/Using_CSS_custom_properties) 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.
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.
