I want to use model data in CSS

Your PHP variables are not available inside your CSS. You either need to add the CSS dynamically or set the styles inline. Here are some examples

<style>
.profile-background {
    background: url("../storage/backgrounds/$user->background_image") no-repeat;
}
</style>

Or using inline styling

<div style="background: url("../storage/backgrounds/$user->background_image") no-repeat;"></div>

I would probably pick the inline styling if it's just the background ;)

Thank you