We haven’t seen a huge demand for adding dynamic backgrounds ie. lots of different images being used across multiple pages or templates. But we’re always looking for feedback for improvement so i can add to the list for us to look into. Thanks for the suggestion.
The CSS can be added using a hook using the body class e.g.
<style type="text/css">
body {
background-image: url('full_url_to_image/image.jpg');
background-size: cover;
}
</style>