You have all the choice in the world. Use whatever tech you want (browser js, server side templating, client side xslt, etc) to have an attribute on the body el

You have all the choice in the world. Use whatever tech you want (browser js, server side templating, client side xslt, etc) to have an attribute on the body element indicating opt-in status, and use that in your media query to target only the users who want it. With the css solution it’s doable on a site that has no javascript.