Emptytheme: experiment with duotone

[MaggieCabrera]

NOT FOR MERGING: This PR is just an experiment to highlight what can be done with theme.json.

Add this content to a page:

<!-- wp:cover {"url":"http://experiments.local/wp-content/uploads/2021/07/guy-man_FSXSYPD4ZV.jpg","id":108,"dimRatio":0,"contentPosition":"bottom left","align":"full","style":{"color":{"duotone":["#000","#BFF5A5"]}}} -->
<div class="wp-block-cover alignfull has-custom-content-position is-position-bottom-left"><img class="wp-block-cover__image-background wp-image-108" alt="" src="http://experiments.local/wp-content/uploads/2021/07/guy-man_FSXSYPD4ZV.jpg" data-object-fit="cover"/><div class="wp-block-cover__inner-container"><!-- wp:paragraph {"align":"center","placeholder":"Write title…","textColor":"green","fontSize":"large"} -->
<p class="has-text-align-center has-green-color has-text-color has-large-font-size">Ready to skate?</p>
<!-- /wp:paragraph -->

<!-- wp:buttons -->
<div class="wp-block-buttons"><!-- wp:button -->
<div class="wp-block-button"><a class="wp-block-button__link">SIGN UP</a></div>
<!-- /wp:button --></div>
<!-- /wp:buttons --></div></div>
<!-- /wp:cover -->

<!-- wp:paragraph -->
<p></p>
<!-- /wp:paragraph -->

Without theme.json	With theme.json