Fauzia Siddique enhanced the Highlighter API in the DefinitelyTyped/DefinitelyTyped repository by extending the highlightClassName property to accept a Record<string, string>, enabling dynamic styling and the application of multiple CSS classes. She approached this by updating TypeScript typings to improve type safety and developer ergonomics, and added comprehensive test coverage to demonstrate and safeguard the new functionality. Her work focused on supporting consistent UI theming across projects, leveraging both React and TypeScript. Through careful API design, test-driven development, and collaborative pull request review, Fauzia delivered a targeted feature that increases theming flexibility without introducing regressions or breaking changes.