popup.css

Cascading Style Sheets file Reference

A .css file contains style rules in the Cascading Style Sheets language. The rules control the appearance of web pages: colors, fonts, spacing, and position. A web browser applies the rules to HTML elements through selectors. One stylesheet can serve many pages, which keeps a site uniform. CSS is a standard of the W3C.

A file extension is only a part of the file name. It does not give a guarantee about the format or the content of a file. But on Windows, the extension selects the program that opens the file.