The file extension for CSS files is
To create a new CSS file in your text editor, create a new file using the menu or keyboard shortcut and then save it. In the dialog that pops up, give it a name and add the
.css file extension at the end. As an example, you could name it
If the file name contains multiple words, use dashes to separate the words instead of spaces, like
First, create a CSS file in your project folder called
main.css. Then create a second CSS file in this folder with a custom name.
CSS basics on MDN