How to override supersized.shutter.css
To alter the value returned from an implementation of hook_libraries_info(), a module or a theme needs to implement hook_libraries_info_alter(). As with other alter hooks, there isn't a way to override the value returned from a specific module, so the hook name is the usual one: modulename_library_info_alter() or themename_library_info_alter().

As side note, you code contains also an error: The parameter is $variables, but the code is using $libraries.