Why my theme’s css not working on another site

This sounds strange if you have tried all these methods correctly and properly.
I think you have to check these points:

  • Purge cache in the whole website if you use a plugin for that
  • Check your css file name if is written correctly, and is the correct path

To solve this problem definitely and quickly, i recommend you to use a child theme generator plugin, to achieve this task. One of the best option is a plugin called Child Theme Configurator which you can find at Child Theme Configurator

Hope this help you.