*@Scott Reinmuth*, I think you misdirection about those tutorials, yes you use [`parse_request`][1] method.

If you follow, this code is a hook with function and you need to put in **functions.php**

    <?php
    	add_action( 'parse_request', 'my_custom_wp_request' );
    	function my_custom_wp_request( $wp ) {
    		if ( !empty( $_GET['my-custom-content'] ) && $_GET['my-custom-content'] == 'css' ) {
    		# get theme options
    		header( 'Content-Type: text/css' );
    ?>
    a {color: <?php echo get_option('some_other_option'); ?> !important;}
    <?php
    		exit; 
    		}
    	}
    ?>

And in **header.php** with this code ( without file **/css/theme_styles.php** ) to make a request with parameter `my-custom-content` and value `css`. Thats why we need a hook with function to make it work.

    <link rel='stylesheet' type='text/css' href="<?php bloginfo( 'url' ); ?>/?my-custom-content=css" />

we done here.

----------


BUT if you need to use css in out side function, then you create **custom-css.php**. And your function in **functions.php** will be like this:

    add_action( 'parse_request', 'my_custom_wp_request' );
    function my_custom_wp_request( $wp ) {
    	if ( !empty( $_GET['my-custom-content'] ) && $_GET['my-custom-content'] == 'css' ) {
    	# get theme options
           header( 'Content-Type: text/css' );
           require dirname( __FILE__ ) . '/css/custom-css.php';
           exit;
    	}
    }   	

and inside your **custom-css.php**

    a {
	  color: <?php echo get_option('some_other_option'); ?> !important;
    } 

In **header.php** still the same above.

> IMHO, this method not recommended. Just use static css file, find another way to make it your css customizable.

  [1]: https://developer.wordpress.org/reference/hooks/parse_request/