>>Also, enable Safari's Develop menu from Preferences - Advanced & use the error console to see if Safari is ignoring your CSS for some reason.
>>>Do not see error console - that might be a matter of the release level of Safari.
Under 'Advanced' tab, StyleSheet shows 'None selected."
The Advanced tab of Safari Preferences includes an option to 'Show Develop menu in menu bar'.
Once you've done that, error console is available in the Develop menu.
>>>Activity does not show anything but the page itself.
That suggests that the way the CSS is linked isn't being recognised.
CSS How to