CSS has developed significantly over the years to meet the demands of modern UI design, emphasizing the importance of theming, which allows for the styling of websites to vary according to context while maintaining visual coherence. This process involves using CSS variables, also known as custom properties, to define and reuse values, thereby enhancing flexibility and efficiency in web design. The article illustrates how developers can create theme-aware applications by implementing CSS variables and JavaScript, allowing for real-time theme switching, such as between dark and light modes. It highlights the benefits of using local storage to remember user preferences, ensuring a consistent experience upon subsequent visits. Additionally, the text discusses the advantages of CSS variable inheritance and local scoping, which facilitate the creation of adaptable and visually appealing websites. Moreover, LogRocket is introduced as a tool for monitoring and improving digital experiences, offering features like session replay and automated user struggle identification.