Overriding css file in theme.info not working as expected

Question

I am trying to remove nice menus css, as per the notes in my Zen theme .info file. I have added the below to the info file, and I haven't provided replacements in my theme so they should be removed.

stylesheets[all][] = nice_menus.css
stylesheets[all][] = nice_menus_default.css

However they are still being imported. Cache cleared.

Thanks

EDIT: I'm aware this can be achieved using hooks, but would like to do it at theme .info file level, unless I am mistaken and it is not possible to remove contrib module .css in the theme .info file.