Child Themes « WordPress Codex: "The required style.css file
style.css is the one and only required file in a child theme. It provides the information header by which WordPress recognizes the child theme, and it replaces the style.css of the parent.
As with any WordPress theme, the information header must be at the top of the file, the only difference being that in a child theme the Template: line is required, so that WordPress knows which theme is the parent of the child theme."
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