bootstrap-customize

Installation

SKILL.md

Bootstrap 5.3 Customization

Bootstrap 5.3 provides powerful customization through Sass variables, CSS custom properties, and a comprehensive theming system including built-in color modes.

Customization Methods

1. CSS Variables (Runtime Customization)

Bootstrap's docs call these "CSS variables" (technically CSS custom properties). Modify styles without recompiling by overriding at any level:

/* Global override */
:root {
  --bs-primary: #0074d9;
  --bs-primary-rgb: 0, 116, 217;
  --bs-body-font-family: 'Inter', sans-serif;
}

/* Component-level override */
.my-card {
  --bs-card-bg: #f0f0f0;
  --bs-card-border-color: transparent;
}

/* Local scope override */
.custom-button {
  --bs-btn-bg: #custom-color;
  --bs-btn-border-color: #custom-color;
  --bs-btn-hover-bg: #darker-custom;
}

CSS Variable Prefix

Customize the bs- prefix via the $prefix Sass variable to avoid conflicts in projects embedding Bootstrap alongside other frameworks:

$prefix: "myapp-"; // Results in --myapp-primary, --myapp-body-bg, etc.

2. Sass Variables (Compile-time)

For comprehensive theming, override Sass variables before importing Bootstrap:

// 1. Include functions first
@import "bootstrap/scss/functions";

// 2. Override default variables
$primary: #0074d9;
$secondary: #7fdbff;
$font-family-base: 'Inter', sans-serif;
$border-radius: 0.5rem;
$enable-shadows: true;

// 3. Import Bootstrap
@import "bootstrap/scss/bootstrap";

Order matters: Variables must be set after functions but before other Bootstrap imports.

Color System

Theme Colors

Eight default theme colors with Sass maps and CSS properties:

$theme-colors: (
  "primary":   $primary,
  "secondary": $secondary,
  "success":   $success,
  "info":      $info,
  "warning":   $warning,
  "danger":    $danger,
  "light":     $light,
  "dark":      $dark
);

Adding Custom Colors

// Add to theme-colors map
$custom-colors: (
  "custom": #900,
  "brand": #1a73e8
);

// Merge with defaults
$theme-colors: map-merge($theme-colors, $custom-colors);

This generates all utilities: .bg-custom, .text-brand, .btn-custom, etc.

Adding Colors with Dark Mode Support

Adding a color to $theme-colors alone won't generate proper dark mode styles for alerts, badges, and list-groups. Define variants in additional maps:

// Light mode variants
$theme-colors-text: map-merge($theme-colors-text, ("custom": #712cf9));
$theme-colors-bg-subtle: map-merge($theme-colors-bg-subtle, ("custom": #e1d2fe));
$theme-colors-border-subtle: map-merge($theme-colors-border-subtle, ("custom": #bfa1fc));

// Dark mode variants
$theme-colors-text-dark: map-merge($theme-colors-text-dark, ("custom": #e1d2f2));
$theme-colors-bg-subtle-dark: map-merge($theme-colors-bg-subtle-dark, ("custom": #8951fa));
$theme-colors-border-subtle-dark: map-merge($theme-colors-border-subtle-dark, ("custom": #e1d2f2));

Without these definitions, components using *-text-emphasis, *-bg-subtle, and *-border-subtle patterns won't display correctly in dark mode.

Removing Colors

$theme-colors: map-remove($theme-colors, "info", "light");

Warning: The primary, success, and danger keys are required and cannot be removed. They're used by Bootstrap's core styles for links, form validation, and other components. Removing them will cause Sass compilation errors.

Safe to remove: secondary, info, warning, light, dark

Cannot remove: primary, success, danger

Color Modes (Dark/Light)

Bootstrap 5.3 includes built-in dark mode support.

Enabling Color Modes

Set data-bs-theme attribute on <html> or any element:

<!-- Light mode (default) -->
<html lang="en" data-bs-theme="light">

<!-- Dark mode -->
<html lang="en" data-bs-theme="dark">

<!-- Per-component -->
<div data-bs-theme="dark">
  <div class="card">Dark card in light page</div>
</div>

JavaScript Toggle

const toggleTheme = () => {
  const html = document.documentElement;
  const current = html.getAttribute('data-bs-theme');
  html.setAttribute('data-bs-theme', current === 'dark' ? 'light' : 'dark');
};

// With system preference
const prefersDark = window.matchMedia('(prefers-color-scheme: dark)');
document.documentElement.setAttribute('data-bs-theme',
  prefersDark.matches ? 'dark' : 'light'
);

// Listen for system changes
prefersDark.addEventListener('change', (e) => {
  document.documentElement.setAttribute('data-bs-theme',
    e.matches ? 'dark' : 'light'
  );
});

Color Mode Implementation Types

Bootstrap supports two approaches for color mode switching, controlled by the $color-mode-type Sass variable.

Data Attribute Method (Default)

$color-mode-type: data;

Uses the data-bs-theme attribute on HTML elements:

<html data-bs-theme="dark">

Advantages:

Per-component theme control possible
User preferences can override system settings
Works with JavaScript theme toggles
Supports runtime switching without page reload

Use when: Building a theme picker UI or needing granular control over individual components.

Media Query Method

$color-mode-type: media-query;

Uses the prefers-color-scheme media query automatically:

@media (prefers-color-scheme: dark) {
  /* Dark mode styles applied automatically */
}

Advantages:

Automatic system preference detection
No JavaScript required
Simpler implementation
Zero runtime overhead

Disadvantages:

No per-component control
Users cannot override via UI
Requires page-level theming only

Use when: You want automatic system preference detection without a manual toggle.

Custom Color Mode Colors

You can define entirely custom color modes beyond light and dark. See references/sass-variables.md for a complete "blue theme" example using [data-bs-theme="blue"].

Override colors per mode:

// Light mode colors
:root,
[data-bs-theme="light"] {
  --bs-body-bg: #ffffff;
  --bs-body-color: #212529;
}

// Dark mode colors
[data-bs-theme="dark"] {
  --bs-body-bg: #1a1a2e;
  --bs-body-color: #e1e1e1;
  --bs-primary: #6ea8fe;
}

Sass Functions

Bootstrap provides utility functions for color manipulation and accessibility:

tint-color($color, $weight) - Lighten by mixing with white
shade-color($color, $weight) - Darken by mixing with black
shift-color($color, $weight) - Auto tint/shade based on weight sign
color-contrast($color) - Get accessible contrast color for backgrounds

// Generate color scale
$primary-light: tint-color($primary, 40%);
$primary-dark: shade-color($primary, 20%);

// Auto-contrast text
.custom-badge {
  background-color: $success;
  color: color-contrast($success); // Returns #fff or #212529
}

// Hover states
.btn-custom:hover {
  background-color: shift-color($brand, 15%);
}

Additional functions: escape-svg() for SVG backgrounds, add()/subtract() for safe CSS calc operations.

See references/sass-functions-mixins.md for complete function and mixin reference.

Key Sass Variables

Typography

$font-family-base: system-ui, -apple-system, sans-serif;
$font-family-monospace: SFMono-Regular, Menlo, monospace;
$font-size-base: 1rem;
$font-size-sm: $font-size-base * 0.875;
$font-size-lg: $font-size-base * 1.25;
$font-weight-normal: 400;
$font-weight-bold: 700;
$line-height-base: 1.5;
$headings-font-family: null; // Inherits $font-family-base
$headings-font-weight: 500;

Spacing

$spacer: 1rem;
$spacers: (
  0: 0,
  1: $spacer * 0.25,  // 0.25rem
  2: $spacer * 0.5,   // 0.5rem
  3: $spacer,         // 1rem
  4: $spacer * 1.5,   // 1.5rem
  5: $spacer * 3      // 3rem
);

Border Radius

$border-radius: 0.375rem;
$border-radius-sm: 0.25rem;
$border-radius-lg: 0.5rem;
$border-radius-xl: 1rem;
$border-radius-xxl: 2rem;
$border-radius-pill: 50rem;

Options Flags

$enable-caret: true;
$enable-rounded: true;
$enable-shadows: false;
$enable-gradients: false;
$enable-transitions: true;
$enable-reduced-motion: true;
$enable-smooth-scroll: true;
$enable-grid-classes: true;
$enable-container-classes: true;
$enable-negative-margins: false;
$enable-dark-mode: true;
$color-mode-type: data; // 'data' or 'media-query'

Focus Ring

Bootstrap 5.3 provides variables to customize :focus styles across all components:

$focus-ring-width: .25rem;
$focus-ring-opacity: .25;
$focus-ring-color: rgba($primary, $focus-ring-opacity);
$focus-ring-blur: 0;
$focus-ring-box-shadow: 0 0 $focus-ring-blur $focus-ring-width $focus-ring-color;

Component Customization

Button Customization

// Global button variables
$btn-padding-y: 0.5rem;
$btn-padding-x: 1rem;
$btn-font-size: 1rem;
$btn-border-radius: $border-radius;

// Per-variant customization via CSS
.btn-primary {
  --bs-btn-bg: #0074d9;
  --bs-btn-border-color: #0074d9;
  --bs-btn-hover-bg: #0063b8;
  --bs-btn-hover-border-color: #005aa3;
  --bs-btn-active-bg: #005aa3;
  --bs-btn-active-border-color: #00518f;
}

Card Customization

$card-spacer-y: 1rem;
$card-spacer-x: 1rem;
$card-border-width: 1px;
$card-border-radius: $border-radius;
$card-border-color: rgba(0, 0, 0, 0.125);
$card-bg: $white;
$card-cap-bg: rgba(0, 0, 0, 0.03);

Form Customization

$input-padding-y: 0.375rem;
$input-padding-x: 0.75rem;
$input-font-size: 1rem;
$input-border-radius: $border-radius;
$input-border-color: $gray-400;
$input-focus-border-color: tint-color($primary, 50%);
$input-focus-box-shadow: 0 0 0 0.25rem rgba($primary, 0.25);

Component Architecture

Bootstrap uses a base-modifier pattern for component styling. Understanding this architecture helps you customize existing components and create new ones that integrate seamlessly.

Base and Modifier Classes

Components start with a base class providing core styling, then modifier classes add variants:

<!-- Base class provides core styling -->
<button class="btn">Base button</button>

<!-- Modifier classes add variants -->
<button class="btn btn-primary">Primary</button>
<button class="btn btn-lg">Large</button>
<button class="btn btn-primary btn-lg">Primary Large</button>

Variant Generation with @each Loops

Bootstrap generates color variants by iterating over $theme-colors:

// How Bootstrap generates .alert-primary, .alert-danger, etc.
@each $state in map-keys($theme-colors) {
  .alert-#{$state} {
    --#{$prefix}alert-color: var(--#{$prefix}#{$state}-text-emphasis);
    --#{$prefix}alert-bg: var(--#{$prefix}#{$state}-bg-subtle);
    --#{$prefix}alert-border-color: var(--#{$prefix}#{$state}-border-subtle);
  }
}

Key insight: Adding a color to $theme-colors automatically generates variants for alerts, buttons, badges, list-groups, and more.

Custom Component with Variants

Apply the same pattern for consistent custom components:

// Base class
.callout {
  padding: var(--bs-callout-padding, 1rem);
  border-left: 4px solid var(--bs-callout-border-color, currentcolor);
  background: var(--bs-callout-bg, transparent);
}

// Generate variants from theme colors
@each $state, $value in $theme-colors {
  .callout-#{$state} {
    --bs-callout-border-color: var(--bs-#{$state});
    --bs-callout-bg: var(--bs-#{$state}-bg-subtle);
  }
}

This generates .callout-primary, .callout-danger, etc., all inheriting from the base .callout styles.

See references/sass-functions-mixins.md for button-variant() and other mixins that follow this pattern.

Creating Custom Components

Use Bootstrap's mixins and functions for consistent components:

.custom-component {
  // Use Bootstrap's spacing
  padding: map-get($spacers, 3);
  margin-bottom: map-get($spacers, 4);

  // Use Bootstrap's colors
  background-color: var(--bs-body-bg);
  color: var(--bs-body-color);
  border: 1px solid var(--bs-border-color);
  border-radius: var(--bs-border-radius);

  // Use Bootstrap's shadows
  @if $enable-shadows {
    box-shadow: $box-shadow-sm;
  }

  // Responsive behavior
  @include media-breakpoint-up(md) {
    padding: map-get($spacers, 4);
  }
}

Optimization

Bootstrap's full bundle is substantial. Use these strategies to reduce production bundle size.

Lean Sass Imports

Import only the components you use:

// Required core
@import "bootstrap/scss/functions";
@import "bootstrap/scss/variables";
@import "bootstrap/scss/variables-dark";
@import "bootstrap/scss/maps";
@import "bootstrap/scss/mixins";
@import "bootstrap/scss/root";

// Optional - import only what you use
@import "bootstrap/scss/reboot";
@import "bootstrap/scss/type";
@import "bootstrap/scss/containers";
@import "bootstrap/scss/grid";
@import "bootstrap/scss/buttons";

See examples/selective-imports.scss for a complete minimal build example.

Lean JavaScript Imports

Tree-shake Bootstrap JavaScript by importing individual plugins:

// Instead of importing everything
// import * as bootstrap from 'bootstrap';

// Import only what you need
import Modal from 'bootstrap/js/dist/modal';
import Dropdown from 'bootstrap/js/dist/dropdown';
import Collapse from 'bootstrap/js/dist/collapse';

// Initialize manually
const modal = new Modal('#myModal');

Note: Some plugins have dependencies (e.g., Dropdown requires Popper.js). Check plugin documentation.

Remove Unused CSS with PurgeCSS

PurgeCSS removes unused styles from your production build:

// postcss.config.js
module.exports = {
  plugins: [
    require('@fullhuman/postcss-purgecss')({
      content: ['./src/**/*.html', './src/**/*.js'],
      // Safelist Bootstrap's dynamic classes
      safelist: {
        standard: [/^modal/, /^show/, /^fade/, /^collapse/, /^offcanvas/],
        deep: [/^tooltip/, /^popover/, /^bs-/]
      }
    })
  ]
}

Bootstrap dynamically adds classes like show, fade, collapsing. Always safelist these patterns.

Autoprefixer Configuration

Configure browser support to avoid unnecessary vendor prefixes:

# .browserslistrc
>= 0.5%
last 2 major versions
not dead
not Explorer <= 11

This is Bootstrap's default configuration. Adjust based on your audience.

Production Best Practices

Minification: Use minified distribution files in production:

bootstrap.min.css (not bootstrap.css)
bootstrap.bundle.min.js (not bootstrap.bundle.js)

Compression: Enable gzip or Brotli on your server:

# nginx.conf
gzip on;
gzip_types text/css application/javascript;

Non-blocking Loading: Use defer for non-critical scripts:

<!-- Critical CSS in head -->
<link rel="stylesheet" href="bootstrap.min.css">

<!-- Scripts at end of body with defer -->
<script src="bootstrap.bundle.min.js" defer></script>

HTTPS: Always serve Bootstrap over HTTPS. CDN links require secure connections and modern browsers may block mixed content.

Security Considerations

Content Security Policy (CSP)

Bootstrap uses embedded SVGs in CSS (data: URIs) for consistent cross-browser styling. If your application has a strict Content Security Policy, some components may not display correctly.

Affected components:

Accordion (collapse/expand icons)
Carousel (prev/next controls)
Close button (X icon)
Form controls (checkboxes, radios, switches, validation icons)
Navbar toggle (hamburger icon)
Select menus (dropdown arrow)

Symptoms of CSP conflicts:

Form checkboxes don't show checkmarks
Accordions missing expand/collapse icons
Navbar hamburger menus appear broken
Select dropdowns lack arrows

Solutions:

Replace with local assets - Use the escape-svg() function with locally hosted SVG files:

// Custom checkbox with local SVG instead of data: URI
$form-check-input-checked-bg-image: url("/assets/icons/check.svg");

Adjust CSP - If appropriate for your security requirements, allow the inline SVG data URIs in your policy
Use Bootstrap Icons font - Replace SVG icons with the Bootstrap Icons font variant

For strict CSP environments, audit Bootstrap's SVG usage during customization planning.

See: Bootstrap CSP Documentation

Additional Resources

Reference Files

references/sass-variables.md - Complete Sass variable reference
references/sass-functions-mixins.md - Complete function and mixin reference

Example Files

examples/color-mode-toggle.html - Complete dark/light mode toggle with system preference detection
examples/custom-theme-demo.html - Runtime CSS variable theming without Sass compilation

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Jan 22, 2026

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