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My parent theme is “Twenty Eleven” and “Graphene” is considered to be a child theme, right?
Nope. Graphene theme is not a child theme of any other theme.
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If I need to change the tagline font and color (as an example), and the option is not within the “Graphene” style.css file, then I have to create some kind of additional child page?
If that’s all you wanna change, you don’t need a child theme for that. You can modify it right from the theme’s options page.