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 anymore; 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.
### 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've downloaded our source files or are using package manager, you'll have a file structure that looks like this:
{% highlight plaintext %}
your-project/
├── scss
│ └── custom.scss
└── node_modules/
└── bootstrap
├── js
└── scss
{% endhighlight %}
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.
{% highlight scss %}
// Custom.scss
// Option A: Include all of Bootstrap
@import "node_modules/bootstrap/scss/bootstrap";
{% endhighlight %}
{% highlight 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";
{% endhighlight %}
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.
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.
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 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.
For example, to modify an existing color in our `$theme-colors` map, add the following to your custom Sass file:
Customize Bootstrap 4 with our built-in custom variables file and easily toggle global CSS preferences with new `$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 our `_variables.scss` file.
| `$spacer` | `1rem` (default), or any value > 0 | Specifies the default spacer value to programmatically generate our [spacer utilities]({{ site.baseurl }}/docs/{{ site.docs_version }}/utilities/spacing/). |
| `$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. |
| `$enable-transitions` | `true` (default) or `false` | Enables predefined `transition`s on various components. |
| `$enable-hover-media-query` | `true` or `false` (default) | ... |
| `$enable-grid-classes` | `true` (default) or `false` | Enables the generation of CSS classes for the grid system (e.g., `.container`, `.row`, `.col-md-1`, etc.). |
| `$enable-print-styles` | `true` (default) or `false` | Enables styles for optimizing printing. |
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, available as Sass variables and a Sass map in our `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.
<divclass="row">
{% for color in site.data.colors %}
{% unless color.name == "white" or color.name == "gray" or color.name == "gray-dark" %}
[Color utility classes]({{ site.baseurl }}/docs/{{ site.docs_version }}/utilities/colors/) are also available for setting `color` and `background-color`.
{% callout info %}
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.
{% endcallout %}
### 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 our `scss/_variables.scss` file.
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.
