Luma

This is actually a default demo theme of Magento 2 itself, which again extends from Magento/blank which extends from the core. But more often, the name Luma refers to the entire stack: XML layout generates a tree-structure of blocks and containers, which eventually render via PHTML templates. The server-side rendered HTML is enriched with CSS (compiled from LESS source files) and JavaScript (RequireJS, KnockoutJS, jQuery).