NPM run build error related to pong.css

[inkiltie]

Describe the bug
When I run npm run build I get this:

> Build error occurred
Error: Build optimization failed: found page without a React Component as default export in 
pages/pong.css

See https://nextjs.org/docs/messages/page-without-valid-component for more info.

The obvious solution is to place pong.css content directly in _app.js, but according to above link, every file that is not exporting React component (export default '') should be placed outside /pages folder.

To Reproduce
Steps to reproduce the behavior:
Run npm run build

Desktop (please complete the following information):

• OS: MacOS 11.4
• Browser Chrome
• Version 90.0.4430.212