Enqueue a file that’s not js or css

Question

You could also look to create a special CSS file that you setup an htaccess rewrite rule for. For example

wp-content/yourtheme/custom.css

And then add a rule in your htaccess:

RewriteEngine On
RewriteRule ^/wp-content/yourtheme/custom.css$ /wp-content/yourtheme/styles.php [L]

You can then call wp-load inside this styles.php file, do any sort of logic required, before outputting your custom css rules:

<?php header("Content-type: text/css; charset: UTF-8"); ?>
p {
   color: black;
}