How can I call a thumbnail/featured image to be a background image?

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This is not a WordPress related, Anyway here is tip –

You can not call a <?php .. ?> function into your style.css file. Instead set the background inline just as shown in this example, and apple styling.

<div style="background: url('<?php wp_get_attachment_image_src(); ?>');">
   // blah blah ...
</div>