How to overriding specific Omega_Kickstart CSS files in a subtheme?
A few things you may want to check (since none of these items are incuded in the question):


make sure you cleared the cache (if not that explains what you experienced).
make sure the file permissions are correct (if not the webserver will not be aware of the newly created subtheme).
make sure you enabled the sub-theme (only creating it is not sufficient).