Placement of

in the tutorial for building a theme there's instructions to add <h1 style="color:{{ config.themes.antimatter.color }}">{{ config.themes.antimatter.info }}</h1> to the theme, but it doesn't specify where. I tried putting it in the .yaml file (didn't go there) and into the page.md file (didn't go there either). Where does it go?

You would put that in a twig template somewhere. Really this is just an example of how to configure a setting, then use that setting to affect rendering.