Cause: I want to write using less, so I have to configure the loader for less by releasing webpack using the eject command. Using the NPM start command, the style of less is normally resolved. When the project is almost finished, NPM run build is prepared. After deployment, the style is all gone. The whole damn thing hurts.

After a whole morning, I finally found the reason

When using eject to release webpack when configured, the start command will change between build and eject. I want to use less after creat-react-app

Solution: