Currently the Featured image is only available within the Header Element.
It will be added to the Block Element in the next update however.
You can of course get the Featured Image URL using the get_the_post_thumbnail_url
function:
https://developer.wordpress.org/reference/functions/get_the_post_thumbnail_url/
Then simply hook into the WP Head to output the styles for a custom-class
eg.
<?php
$featured_img_url = get_the_post_thumbnail_url($post->ID, 'full');
if ($featured_img_url) {
$style = printf(
'<style>.custom-class {background: url(%1$s);}</style>',
$featured_img_url
);
return $style;
}
?>