Bootstrap/docs/getting-started/customize.md

layout	title
page	Customize CSS

New to Bootstrap 4, variable-based customization is a way for folks to quickly enable or disable global styles across all of Bootstrap's CSS. Simply update a variable's value and recompile using our Gruntfile.

Available variables

You can find and customize these variables in our _variables.scss file.

Variable	Values	Description
$spacer	1rem (default), or any value > 0	Specifies the default spacer value for our spacer utilities.
$enable-flex	true or false (default)	Swaps float and display: table styles for display: flex.
$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 various components.
$enable-transitions	true (default) or false	Enables predefined transitions on various components.
$enable-hover-media-query	true or false (default)	...