A robust approach to handling app typography using theme_extensions for large scale apps with SOLID principles. PS: Open to questions, suggestions and improvements.

app_typography.dart

import 'package:flutter/material.dart';

abstract class AppTypography {
  AppTypography({
    required this.name,
    required this.regular,
    required this.medium,
    required this.semiBold,
    required this.bold,
  });

  final String name;

  final TextStyle regular;
  final TextStyle medium;
  final TextStyle semiBold;
  final TextStyle bold;

  AppTypography copyWith({
    TextStyle? regular,
    TextStyle? medium,
    TextStyle? semiBold,
    TextStyle? bold,
  });

  AppTypography lerp(covariant AppTypography other, double t);
}

header_3.dart

import 'package:flutter/material.dart';
import 'app_typography.dart';

class Header3 extends AppTypography {
  Header3({
    TextStyle? regular,
    TextStyle? medium,
    TextStyle? semiBold,
    TextStyle? bold,
  }) : super(
          name: 'Header3',
          regular: regular ?? _base,
          medium: medium ?? _base.copyWith(fontWeight: FontWeight.w500),
          semiBold: semiBold ?? _base.copyWith(fontWeight: FontWeight.w600),
          bold: bold ?? _base.copyWith(fontWeight: FontWeight.w700),
        );

  static const TextStyle _base = TextStyle(fontSize: 32);

  @override
  Header3 copyWith({
    TextStyle? regular,
    TextStyle? medium,
    TextStyle? semiBold,
    TextStyle? bold,
  }) {
    return Header3(
      regular: regular ?? this.regular,
      medium: medium ?? this.medium,
      semiBold: semiBold ?? this.semiBold,
      bold: bold ?? this.bold,
    );
  }

  @override
  Header3 lerp(covariant AppTypography other, double t) {
    if (other is! Header3) return this;

    return Header3(
      regular: TextStyle.lerp(regular, other.regular, t),
      medium: TextStyle.lerp(medium, other.medium, t),
      semiBold: TextStyle.lerp(semiBold, other.semiBold, t),
      bold: TextStyle.lerp(bold, other.bold, t),
    );
  }
}

typography_extension.dart

import 'package:flutter/material.dart';
import 'app_typography';

@immutable
class TypographyExtension extends ThemeExtension<TypographyExtension> {
  const TypographyExtension({
    required this.heading3,
    required this.heading4,
    required this.heading5,
    required this.heading6,
    required this.subHeading,
    required this.body,
    required this.subText,
    required this.caption,
  });

  final AppTypography heading3;
  final AppTypography heading4;
  final AppTypography heading5;
  final AppTypography heading6;
  final AppTypography subHeading;
  final AppTypography body;
  final AppTypography subText;
  final AppTypography caption;

  @override
  TypographyExtension copyWith({
    AppTypography? heading3,
    AppTypography? heading4,
    AppTypography? heading5,
    AppTypography? heading6,
    AppTypography? subHeading,
    AppTypography? body,
    AppTypography? subText,
    AppTypography? caption,
  }) {
    return TypographyExtension(
      heading3: heading3 ?? this.heading3,
      heading4: heading4 ?? this.heading4,
      heading5: heading5 ?? this.heading5,
      heading6: heading6 ?? this.heading6,
      subHeading: subHeading ?? this.subHeading,
      body: body ?? this.body,
      subText: subText ?? this.subText,
      caption: caption ?? this.caption,
    );
  }

  @override
  TypographyExtension lerp(
      ThemeExtension<TypographyExtension>? other, double t) {
    if (other is! TypographyExtension) return this;

    return TypographyExtension(
      heading3: heading3.lerp(other.heading3, t),
      heading4: heading4.lerp(other.heading4, t),
      heading5: heading5.lerp(other.heading5, t),
      heading6: heading6.lerp(other.heading6, t),
      subHeading: subHeading.lerp(other.subHeading, t),
      body: body.lerp(other.body, t),
      subText: subText.lerp(other.subText, t),
      caption: caption.lerp(other.caption, t),
    );
  }
}