I am trying to understand the new layout of Magento themes and layout. I am currently using the Magento Luma theme. The theme works well but I would like to enable a background image. I have attempted to add to add to the CSS in the pub/static/frontend/Magento/luma/en_US/css/styles-l.css file but it gets ignored. Am I missing something? Is there a better way to be doing this?