A CSS Less developer utilizes the dynamic preprocessor style sheet language, Less, to enhance CSS's usability. Their duties include creating variables, mixins, functions, and operations to write cleaner, more efficient CSS. They develop reusable, maintainable, and extensible CSS codes for web applications. They also work on improving website responsiveness and speed, optimizing the CSS performance. Additionally, they troubleshoot and fix issues related to CSS and ensure cross-browser compatibility. They collaborate with other team members, like web designers and developers, to enhance user interface and overall user experience. They continuously learn new techniques and updates in the Less language.