silverstripe-webpack/app/client/src/scss/_bootstrap_variables.scss

// bootstrap minify bugfix:
$navbar-dark-toggler-icon-bg: none;
$navbar-light-toggler-icon-bg: none;

// IE > 9
$enable-flex: true;

@import "~bootstrap/scss/functions";
@import "~bootstrap/scss/variables";
@import "~bootstrap/scss/mixins";