Sets up theme defaults and registers support for various WordPress features.

Note that this function is hooked into the after_setup_theme hook, which
runs before the init hook. The init hook is too late for some features, such
as indicating support for post thumbnails.

Create your own twentysixteen_setup() function to override in a child theme.

$GLOBALS['content_width'] = apply_filters( 'twentysixteen_content_width', 840 );