Definition: Theme.js is a JavaScript library used in building static site generators for React, Vue, Angular, and similar applications. It's best known as a component that provides a theme for your site's content and style. The specific usage of Theme.js can vary depending on the project you are working on. The Theme.js file contains logic to apply styles to the content of the site using the framework's CSS or JSX language. This includes applying class, font-size, color schemes, margins, paddings, spacing, etc. Each theme has its own set of settings and rules that control these features. Themes can be applied across entire sites, allowing you to maintain consistency in your design style while supporting local branding and site-specific requirements. To use Theme.js, you create a new component with a template and theme class. Then, through the lifecycle hook, you apply specific styles for your component using Theme.js' methods such as applyClasses() or applyStyles(), which can be applied to HTML elements or renderChildren(). The `applyClasses()` method is used to add classes to the component's style, while the `applyStyles()` method applies those classes to the HTML element that contains it. For example, in a React application, you could use Theme.js to create a theme that uses a custom font and color scheme: ```jsx import { Component } from 'react'; import './theme.css'; class MyApp extends Component { static getInitialProps = ({ location }) => ({ currentTheme: 'light', }); render() { const currentThemeClass = this.props.currentTheme === 'dark' ? 'light-theme' : 'dark-theme'; return (