My b2evolution Version: 3.3.3-stable

I have more than one blogs on my system. All of them use the same skin (vasitude). I would like to make small customization for each blog (say have a different header background picture). Is it possible?

On the admin page, I see this option "Allow customized blog CSS file" which talks about doing it by creating a file name style.css in the blog's media folder. Does it work? I tried it, but it doesn't seem to work.

Can anyone help?

Thanks in advance.
ET