Theme support popular free Newsletters plugin – MailPoet Newsletters. This plugin is recomended, but not required. If you dont need it, dont install it.
After installing this plugin navigate to WordPress dashboard → MailPoet → Settings → Forms and edit or create new form. You can add this form as widget in widget area or as shortcode in posts or pages (via our shortcode generator)
Theme styling only subscribe form (widget or/and shortcode), everythig else is under plugin control.