Nowadays, WordPress is not only used for weblogs, but it can also be used as a content management system for building a website.
The WordPress system, written in PHP, has been split into the core system, plugins, and themes. The plugins add additional functionalities to the system, and the themes handle the look and feel of a website built with WordPress. They work independently of each other and are also independent of the theme. The theme does not depend on plugins. WordPress themes define the global CSS for a website, but every plugin can also add its own CSS code.
The WordPress theme developers can use Less to compile the CSS code of the themes and the plugins.
Sage is a WordPress starter theme. You can use it to build your own theme. The theme is based on HTML5 Boilerplate (http://html5boilerplate.com/) and Bootstrap. Visit the Sage theme website at https://roots.io/sage/. Sage can also be completely built using Gulp.