the main purpose of SSL is security.
So, what is security ?
Securit means you don't want other person/devices to see the content of your traffic.
If nobody can see the content, this includes the CSS.
So, the CSS is unable to see the URL which is part of the content.
Your solution can't work.
Unless you install an ssl module with the key of the server so the module can decrypt the traffic.
Regards,
Gilles.