wp-newbee on "new version 3.6, style.css won't wirk in child theme, override the whole thing"

hello,
anybody updated wordpress to 3.6 version?
I started a new project using 3.6, Created a child theme to test the new version, I used the twentytwelve as a base (custom headr,etc.), I created a style.css, but it will completely override twentytwelve style sheet. It won't keep the original theme style.
Any thoughts?

Thank you