Default wordpress theme need a two necessarily files style.css & index.php.

Screenshot.png for privew on admin page

get_stylesheet_uri(); function for stylesheet url

define('FS_METHOD', 'direct'); Allow install plugins using filesystem instead of FTP parameters

add_theme_support() for thumbnails and title etc.

add_action() Actions are the hooks that the WordPress core launches at specific points during execution, or when specific events occur.

add_filter() filter hooks to allow plugins to modify various types of internal data at runtime

add_shortcode() custom shortcode

get_post_format() return type of post