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A design system keeps your app's look consistent across phones, foldables, and tablets. Two pillars hold it together: typography (how text looks at every size) and component theming (how buttons, cards, and inputs are styled once and reused everywhere).

In Material 3, both live inside ThemeData. Instead of styling each widget by hand, you define the rules centrally and let widgets inherit them. This lesson shows how to scale text responsively and customize component themes for consistent cross-device branding.

• TextTheme carries the type scale (display, headline, title, body, label).
• Component themes (e.g. ElevatedButtonThemeData) shape each widget family.

Material 3 defines a type scale of 15 named styles grouped into 5 roles. Each role has Large, Medium, and Small variants:

• displayLarge/Medium/Small — big, expressive hero text.
• headlineLarge/Medium/Small — section headers.
• titleLarge/Medium/Small — list and dialog titles.
• bodyLarge/Medium/Small — paragraphs and main content.
• labelLarge/Medium/Small — buttons, captions, chips.

You read these from context anywhere in your widget tree, so a heading always matches the same scale no matter where it appears.

To brand your typography, override the textTheme on ThemeData. A clean approach is to start from a base text theme and copyWith only the styles you want to change. This keeps the rest of the Material defaults intact.

Notice how each style is a TextStyle with explicit fontSize, fontWeight, and letterSpacing. Defining them in one place guarantees consistency across every screen.

Hard-coded font sizes look great on one device but cramped or oversized on another. Use MediaQuery to read the screen width and pick a scale factor. A common pattern: clamp a base size between a minimum and a maximum so text never gets absurdly small or large.

Here we compute a responsive headline size from the available width, then apply it on top of the theme's style with copyWith.

Accessibility matters: users can enlarge system text. In modern Flutter, read this via MediaQuery.textScalerOf(context), which returns a TextScaler. Avoid disabling it outright; instead clamp it so very large settings don't break your layout while still honoring the user's preference.

Wrap a subtree in MediaQuery with a clamped scaler to bound the growth.

The scaling logic itself is plain Dart and easy to unit-test in isolation, no Flutter needed. Below is a small function that maps a screen width to a font size with a linear ramp between two breakpoints, then clamps the result. Testing this pure logic separately keeps your widgets thin.

Beyond text, Material 3 lets you theme entire widget families centrally. Each has a dedicated *ThemeData slot on ThemeData:

• elevatedButtonTheme, filledButtonTheme, textButtonTheme
• cardTheme, chipTheme, appBarTheme
• inputDecorationTheme for text fields

Set them once and every matching widget inherits the styling. This is the heart of a design system: change the brand in one file, see it everywhere.

Use ElevatedButtonThemeData with a ButtonStyle to standardize padding, shape, and text style for every ElevatedButton in the app. Tying the button's label style back to labelLarge keeps it aligned with your type scale.

Because the theme supplies these defaults, individual buttons stay clean: ElevatedButton(onPressed: ..., child: Text('Save')) with no inline styling.

Cards and text fields are everywhere in mobile UIs, so theming them pays off fast. CardThemeData controls elevation, shape, and margin; InputDecorationTheme standardizes borders, fills, and label behavior for every TextField.

With these in place, a form looks consistent across the app without repeating decoration code on each field.

Now combine typography and component themes into a single ThemeData. Pairing it with a ColorScheme.fromSeed gives you a coherent Material 3 palette. Pass this theme to MaterialApp(theme: ...) and the whole app picks it up.

This one object is your design system's source of truth: brand colors, type scale, and component styling, all together.

Once the theme is set, widgets should pull styling from it rather than hard-coding values. Use Theme.of(context) to access the textTheme, colorScheme, and any component theme.

This makes your widgets theme-agnostic: switch to a dark theme or rebrand, and they update automatically because they never assumed a specific color or size.

You built the typography and component foundation of a Material 3 design system:

• Type scale: the 15 named styles (display, headline, title, body, label) read via Theme.of(context).textTheme.
• Custom TextTheme: override fonts and sizes centrally with copyWith.
• Responsive sizing: derive sizes from MediaQuery width and clamp them, and respect (but bound) the user's textScaler for accessibility.
• Component themes: ElevatedButtonThemeData, CardThemeData, and InputDecorationTheme style entire widget families once.
• Single source of truth: assemble everything into one ThemeData with ColorScheme.fromSeed, then read it in theme-agnostic widgets.

The result is consistent cross-device branding that adapts to screen size and user preferences without per-widget styling.