It seems like the setup you've done is correct.
If you encounter any issues, a helpful tactic is to utilize Chrome's development window. Despite any personal preferences towards Chrome, the array of tools available in the development window makes it a valuable resource.
To access the development window in Chrome with your xhtml page, press F12. Proceed to the console tab to check for any potential errors in loading the css file. The issue could stem from permissions or CORS depending on the hosting or access method of the css file.
In the absence of errors in the console, navigate to the sources tab and explore the resource tree until you locate the css file. This will provide detailed information about the resource and offer insights into why it may not be loading.
If necessary, you can experiment with using an absolute path as a workaround. However, it is advisable to stick to relative paths as they are standard practice. Absolute paths should only be considered as a last resort if relative paths prove unsuccessful in resolving the issue quickly.