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CSS Variables

Fonttrio uses CSS custom properties (CSS variables) to provide a flexible, maintainable system for applying font pairings to your project. This approach makes it easy to swap fonts, customize scales, and maintain consistency across your entire application.

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Why CSS Variables?

CSS variables offer several advantages for managing typography:
  • Single source of truth: Define fonts once, use everywhere
  • Easy theming: Switch entire pairings by changing a few variables
  • Dynamic updates: Change typography at runtime without recompiling
  • Scoped customization: Override variables for specific components
  • Framework agnostic: Works with any CSS-based framework

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Core Font Variables

Every Fonttrio pairing defines three core CSS variables for the font families: 
:root {
  --font-heading: var(--font-playfair-display);
  --font-body: var(--font-source-serif-4);
  --font-mono: var(--font-jetbrains-mono);
}
These three variables are then referenced throughout your stylesheets to apply the fonts consistently.

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Variable Structure in Pairings

Each pairing in the Fonttrio registry defines its CSS variables in the cssVars section: 
{
  "name": "pairing-editorial",
  "cssVars": {
    "theme": {
      "--font-heading": "var(--font-playfair-display)",
      "--font-body": "var(--font-source-serif-4)",
      "--font-mono": "var(--font-jetbrains-mono)"
    }
  },
  "css": {
    "h1": { 
      "font-family": "var(--font-heading)", 
      "font-size": "2.25rem", 
      "line-height": "1.2", 
      "letter-spacing": "-0.025em", 
      "font-weight": "700" 
    },
    "h2": { 
      "font-family": "var(--font-heading)", 
      "font-size": "1.875rem", 
      "line-height": "1.25", 
      "letter-spacing": "-0.02em", 
      "font-weight": "600" 
    },
    "h3": { 
      "font-family": "var(--font-heading)", 
      "font-size": "1.5rem", 
      "line-height": "1.3", 
      "letter-spacing": "-0.015em", 
      "font-weight": "600" 
    },
    "h4, h5, h6": { 
      "font-family": "var(--font-heading)", 
      "letter-spacing": "-0.01em" 
    },
    "body, p": { 
      "font-family": "var(--font-body)", 
      "line-height": "1.65" 
    },
    "code, pre": { 
      "font-family": "var(--font-mono)" 
    }
  }
}

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How Variables are Applied

Fonttrio uses a two-layer variable system for maximum flexibility:

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Layer 1: Font Family Variables

Individual fonts define their own CSS variable: 
/* Each font gets its own variable */
--font-playfair-display: 'Playfair Display', serif;
--font-source-serif-4: 'Source Serif 4', serif;
--font-jetbrains-mono: 'JetBrains Mono', monospace;

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Layer 2: Semantic Variables

Pairings reference these font variables through semantic names: 
/* Pairing maps semantic names to specific fonts */
--font-heading: var(--font-playfair-display);
--font-body: var(--font-source-serif-4);
--font-mono: var(--font-jetbrains-mono);
This two-layer approach means you can:
  • Switch pairings by changing only 3 variables
  • Override specific fonts while keeping others
  • Create variations of existing pairings

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Using Variables in Your CSS

Once a pairing is installed, use the semantic variables in your stylesheets: 
/* Headings use the heading font */
h1, h2, h3, h4, h5, h6 {
  font-family: var(--font-heading);
}

/* Body text uses the body font */
body, p, li, td {
  font-family: var(--font-body);
}

/* Code uses the monospace font */
code, pre, kbd, samp {
  font-family: var(--font-mono);
}

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Fonttrio’s Global CSS Setup

Here’s how Fonttrio sets up its own CSS variable system in /home/daytona/workspace/source/app/globals.css:85-98:
app/globals.css
@theme inline {
  --color-bg: var(--bg);
  --color-text: var(--text);
  --color-text-muted: var(--text-muted);
  --color-text-subtle: var(--text-subtle);
  --color-surface-border: var(--surface-border);
  --color-surface-border-strong: var(--surface-border-strong);
  --color-surface: var(--surface);
  --color-surface-hover: color-mix(in oklch, var(--surface) 80%, var(--foreground) 20%);

  --font-display: var(--font-bebas-neue), system-ui, sans-serif;
  --font-sans: system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
  --font-mono: ui-monospace, SFMono-Regular, "SF Mono", Menlo, Consolas, monospace;
}
This shows how Fonttrio extends CSS variables beyond just fonts to include colors and other design tokens.

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Typography-Specific Variables

Beyond font families, you can create variables for the entire typography scale: 
:root {
  /* Font families */
  --font-heading: var(--font-playfair-display);
  --font-body: var(--font-source-serif-4);
  --font-mono: var(--font-jetbrains-mono);
  
  /* Typography scale */
  --font-size-h1: 2.25rem;
  --font-size-h2: 1.875rem;
  --font-size-h3: 1.5rem;
  --font-size-body: 1rem;
  
  /* Line heights */
  --line-height-heading: 1.2;
  --line-height-body: 1.65;
  
  /* Letter spacing */
  --letter-spacing-tight: -0.025em;
  --letter-spacing-normal: 0em;
}

h1 {
  font-family: var(--font-heading);
  font-size: var(--font-size-h1);
  line-height: var(--line-height-heading);
  letter-spacing: var(--letter-spacing-tight);
}

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Scoped Customization

CSS variables can be scoped to specific elements or components: 
/* Global default */
:root {
  --font-heading: var(--font-inter);
  --font-body: var(--font-inter);
}

/* Override for marketing section */
.marketing-hero {
  --font-heading: var(--font-playfair-display);
}

/* Override for documentation */
.docs-content {
  --font-body: var(--font-source-serif-4);
  --line-height-body: 1.75; /* More readable for long-form */
}

/* Both h1 elements use var(--font-heading) but get different fonts */
h1 {
  font-family: var(--font-heading);
}
Scoped variables allow you to use different pairings in different parts of your site without conflicts.

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Dark Mode Support

CSS variables make theme switching trivial. Here’s how Fonttrio handles dark mode in /home/daytona/workspace/source/app/globals.css:7-50:
app/globals.css
:root {
  --bg: #fafafa;
  --text: #0a0a0a;
  --text-muted: #666666;
  --surface-border: #eaeaea;
  /* ...more light mode colors */
}

.dark {
  --bg: #0a0a0a;
  --text: #ededed;
  --text-muted: #888888;
  --surface-border: #1f1f1f;
  /* ...more dark mode colors */
}
The variables automatically update when the .dark class is applied, without touching font definitions.

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Dynamic Updates with JavaScript

You can change CSS variables programmatically: 
// Switch to a different pairing at runtime
function switchPairing(pairing: 'editorial' | 'minimal' | 'brutalist') {
  const root = document.documentElement;
  
  if (pairing === 'editorial') {
    root.style.setProperty('--font-heading', 'var(--font-playfair-display)');
    root.style.setProperty('--font-body', 'var(--font-source-serif-4)');
    root.style.setProperty('--font-mono', 'var(--font-jetbrains-mono)');
  } else if (pairing === 'minimal') {
    root.style.setProperty('--font-heading', 'var(--font-geist)');
    root.style.setProperty('--font-body', 'var(--font-geist)');
    root.style.setProperty('--font-mono', 'var(--font-geist-mono)');
  }
  // Changes apply instantly across the entire site
}

// Adjust font size dynamically
function setFontSize(scale: number) {
  document.documentElement.style.setProperty('--font-size-body', `${scale}rem`);
}

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Fallback Fonts

Always provide fallback fonts for better loading experience: 
:root {
  /* Fonttrio fonts with system fallbacks */
  --font-heading: var(--font-playfair-display), Georgia, serif;
  --font-body: var(--font-source-serif-4), Georgia, serif;
  --font-mono: var(--font-jetbrains-mono), 'Courier New', monospace;
  
  /* Fonttrio uses system fallbacks */
  --font-sans: system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
  --font-mono: ui-monospace, SFMono-Regular, "SF Mono", Menlo, Consolas, monospace;
}

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Tailwind CSS Integration

If you’re using Tailwind, extend your config to use CSS variables:
tailwind.config.js
module.exports = {
  theme: {
    extend: {
      fontFamily: {
        heading: ['var(--font-heading)'],
        body: ['var(--font-body)'],
        mono: ['var(--font-mono)'],
      },
    },
  },
}
Then use in your HTML: 
<h1 class="font-heading text-4xl">Heading</h1>
<p class="font-body text-base">Body text</p>
<code class="font-mono">Code</code>

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Next.js Font Integration

Fonttrio works seamlessly with Next.js font optimization:
app/layout.tsx
import { Playfair_Display, Source_Serif_4, JetBrains_Mono } from 'next/font/google';

const playfair = Playfair_Display({
  subsets: ['latin'],
  variable: '--font-playfair-display',
  display: 'swap',
});

const sourceSerif = Source_Serif_4({
  subsets: ['latin'],
  variable: '--font-source-serif-4',
  display: 'swap',
});

const jetbrainsMono = JetBrains_Mono({
  subsets: ['latin'],
  variable: '--font-jetbrains-mono',
  display: 'swap',
});

export default function RootLayout({ children }) {
  return (
    <html className={`${playfair.variable} ${sourceSerif.variable} ${jetbrainsMono.variable}`}>
      <body>{children}</body>
    </html>
  );
}

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Variable Naming Conventions

Fonttrio follows these naming conventions: 
/* ✅ Good: Semantic, purpose-driven names */
--font-heading
--font-body
--font-mono

/* ❌ Avoid: Font-specific names */
--font-playfair
--font-source

/* ✅ Good: Scale-specific variables */
--font-size-h1
--line-height-body
--letter-spacing-tight

/* ❌ Avoid: Non-descriptive names */
--size-1
--lh-a
--ls-small

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Best Practices

Use Semantic Names: Prefer --font-heading over --font-playfair so you can swap fonts without changing references.
Layer Your Variables: Create base variables for individual fonts, then semantic variables that reference them.
Provide Fallbacks: Always include fallback fonts in case custom fonts fail to load.
Scope Strategically: Use scoped variables for component-level customization without affecting global styles.
Test Everywhere: CSS variables work in all modern browsers, but test your fallbacks in older environments.

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Common Patterns

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Pattern 1: Pairing Switcher

/* Define multiple pairings */
.pairing-editorial {
  --font-heading: var(--font-playfair-display);
  --font-body: var(--font-source-serif-4);
}

.pairing-minimal {
  --font-heading: var(--font-geist);
  --font-body: var(--font-geist);
}

/* Apply to body or root element */
body {
  font-family: var(--font-body);
}

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Pattern 2: Responsive Typography

:root {
  --font-size-h1: 2rem;
}

@media (min-width: 768px) {
  :root {
    --font-size-h1: 2.5rem;
  }
}

h1 {
  font-size: var(--font-size-h1);
}

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Pattern 3: User Preferences

/* Default */
:root {
  --font-size-base: 1rem;
}

/* User selected large text */
body.text-large {
  --font-size-base: 1.25rem;
}

/* All sizes scale proportionally */
body {
  font-size: var(--font-size-base);
}

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Next Steps

Font Pairings

Learn about Fonttrio’s pairing structure

Typography Scale

Understand how typography scales work

Installation

Install Fonttrio and start using CSS variables

Customization

Customize pairings for your project

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