That’s actually not strange, a lot of free and premium themes locate style sheets in sub folders mainly to keep things organized. You may one day come across a theme that has multiple style sheets, to display different colors, responsive/mobile ones, browser specific….
Also I would suggest if you can not to edit in the WordPress editor if you can help it but rather through a program like Coda or Dreamweaver. That way if you mess up you will have some undos and local copies of the site. Just my 2 cents.