Starting with Version 4.1, the Twenty Fifteen theme became the default (and fallback) theme.

It is clean, blog-focused, and designed for clarity. Twenty Fifteen's simple, straightforward typography is readable on a wide variety of screen sizes, and suitable for multiple languages. It was designed using a mobile-first approach, meaning content takes center-stage, regardless of whether the site is accessed by smartphone, tablet, laptop, or desktop computer.

Reference http://codex.wordpress.org/Glossary