A Cascading Style Sheet (CSS) file is used to format/style the contents of a webpage.
You can open/create a CSS file using an Editor e.g. Notepad++ on Windows computers. A CSS file has the extension .css
You can actually open a .css file using any Text Editors, including the default 'Notepad' in Windows computers.
However, you should use web development programs, which provide more advanced features for editing CSS and other programs files.