What is a defining characteristic of themelets in Liferay?

Themelets in Liferay are primarily designed to promote a more modular approach to coding. This characteristic allows developers to create smaller, reusable components that can be combined to form a complete theme. By using themelets, developers can isolate different aspects of a theme, such as styles and functionalities, which enhances both maintainability and scalability. Modular coding through themelets encourages best practices, enabling developers to work more efficiently, as adjustments or enhancements can be made without affecting the entire theme.

The other options do not accurately reflect the nature of themelets. For instance, while themelets are valuable components, they are not mandatory for all themes, meaning that developers can choose whether or not to utilize them based on their project requirements. Additionally, themelets are not limited to only including CSS; they can also encompass other parts of the theme, such as templates or JavaScript. Finally, themelets do not require a specific implementation language; they can be created using the standard languages and frameworks that are compatible with Liferay themes. This flexibility further underscores the modularity and adaptability that themelets bring to the theme development process.