What makes a Wordpress theme an "invalid parent theme"?

Question

In order to overwrite some CSS of an existing Wordpress theme, I would like to create a child theme of it as described on http://codex.wordpress.org/Child_Themes#Example_of_a_basic_Child_Theme .

I created a directory along with the style.css file, where the parent theme's name is defined as "template". So far, so good. When I go to the themes page in Wordpress' administration, I get the message "The [name of parent theme] theme is not a valid parent theme.", and I cannot activate it.

I am wondering how Wordpress finds out if a parent theme is valid or not and if there is a way to make the parent theme a valid parent theme.

In order to find out if the defined parent theme is the problem, I replaced the "template" definition with another theme, which is also installed on the same server, and that worked.

Thanks in advance for your help.

Answer 1

It seems I forgot to mention the fact that caused the problem: My parent theme is already a child theme, and I was not aware of the two generation limit, which means that a child child theme is not possible. :-(

Thanks for your help anyway! Maybe this information should be added on http://codex.wordpress.org/Child_Themes ...