Render CSS from PHP file with access to WP functions

functions.php

// functions.php

add_action("wp_enqueue_scripts", "css_styles", 1000);
function css_styles()
{    

    // load previews
    wp_enqueue_style( 'previews-css', '/previews');
 
 }

// Add a rewrite rule.
add_action( 'init', function () {
    add_rewrite_endpoint( 'previews', EP_ROOT );
});

// For requests to the /previews page, set a queryvar
add_filter( 'request', function ($vars) {
    if (isset($vars["name"]) && $vars["name"] == "previews") {
        $vars["previews"] = true;
    }
    return $vars;
});

// If there is a queryvar, render css instead
add_action( 'template_redirect', function () {
    if (get_query_var("previews")) {
        locate_template( 'includes/previews.php', TRUE, TRUE );
        exit();
    }
});




// includes/previews.php

status_header("200");
header('Content-Type: text/css;charset=utf-8');

WP_Query()...

// echo CSS

Make sure to cache the output CSS; this effectively initializes WP a second time for each request!