I see what I did wrong, I left out the import. The child theme style.css should also include an @import too.
Theme Name: Twenty Ten Child
Theme URI: http: //example.com/
Description: Child theme for the Twenty Ten theme
Author: Your name here
Author URI: http: //example.com/about/
See the @import portion? That will include all of the style CSS info from the parent. Any CSS you include in your child style.css will override the parent theme CSS.
In one of my child themes for TwentyTen I added these lines after the @import (I forgot why I did):
padding: 0 20px;
/* this?: */
That will contain the portions of CSS that I like and override the parent theme CSS for just those portions.
All the data will come from the parent theme unless there is a file with the file name in the child directory. So if you replace just single.php only as above, all the .php files will come from the parent except that one single.php file.
But do I copy the entire file
Yes, copy the entire file.