Hi there,
The CSS file stays inside your Chrome extension. And with the Chrome inspector, you can see if the CSS override, that with the label "injected stylesheet".
For example, this is the accessible link in the Chrome web browser (for my Turn Off the Lights Chrome extension):
chrome-extension://bfbmjmiodbnnpllbbbfblcplfjjepjdn/css/light.css
For you can be at this location:
chrome-extension://YourID/content-script.css
For more information about debugging see this documentation:
Thanks,