A Sass file to @import before importing Bootstrap 4's Reboot file. Uses Bootstrap defaults.

_reboot-variables.scss

// Required mixin
$enable-hover-media-query: false;
@mixin hover-focus {
  @if $enable-hover-media-query {
    &:focus { @content }
    @include hover { @content }
  }
  @else {
    &:focus,
    &:hover {
      @content
    }
  }
}

// Variables
$white: #fff;
$blue: #0275d8;
$gray-dark: #292b2c;
$gray-light: #636c72;
$brand-primary: $blue;
$text-muted: $gray-light;
$font-family-sans-serif: -apple-system, system-ui, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
$font-weight-normal: normal;
$font-weight-bold: bold;

// Reboot settings
$font-family-base: $font-family-sans-serif;
$font-size-base: 1rem; // Assumes the browser default, typically `16px`
$font-weight-base: $font-weight-normal;
$line-height-base: 1.5;
$body-color: $gray-dark;
$body-bg: $white;
$dt-font-weight: $font-weight-bold;
$link-color: $brand-primary;
$link-decoration: none;
$link-hover-color: darken($link-color, 15%);
$link-hover-decoration: underline;
$table-bg: transparent;
$table-cell-padding: .75rem;
$cursor-disabled: not-allowed;