reply to http://wordpress.stackexchange.com/questions/190817/use-scssphp-to-compile-theme-customizer-values-into-scss-files-ready-to-compile/190924?noredirect=1#comment278659_190924

compiler.php

<?php

class Aristath_Dynamic_CSS {

    public $stylesheet_id;

    public function __construct() {
        add_action( 'customize_save_after', array( $this, 'make_css' ) );
    }

    /**
     * Enqueue the dynamic CSS.
     */
    public function enqueue_dynamic_css() {
        wp_enqueue_style( 'dynamic-css', $this->file( 'uri' ) );
    }

    public function make_css() {

        global $wp_filesystem;

        // Initialize the Wordpress filesystem.
        if ( empty( $wp_filesystem ) ) {
            require_once( ABSPATH . '/wp-admin/includes/file.php' );
            WP_Filesystem();
        }

        $content = "/********* Compiled - Do not edit *********/\n" . $this->compile_scss() );

        // Take care of domain mapping
        if ( defined( 'DOMAIN_MAPPING' ) && DOMAIN_MAPPING ) {

            if ( function_exists( 'domain_mapping_siteurl' ) && function_exists( 'get_original_url' ) ) {

                $mapped_domain   = domain_mapping_siteurl( false );
                $mapped_domain   = str_replace( 'https://', '//', $domain_mapping );
                $mapped_domain   = str_replace( 'http://', '//', $mapped_domain );

                $original_domain = get_original_url( 'siteurl' );
                $original_domain = str_replace( 'https://', '//', $original_domain );
                $original_domain = str_replace( 'http://', '//', $original_domain );

                $content = str_replace( $original_domain, $mapped_domain, $content );

            }

        }

        // Strip protocols
        $content = str_replace( 'https://', '//', $content );
        $content = str_replace( 'http://', '//', $content );

        if ( is_writable( $this->file( 'path' ) ) || ( ! file_exists( $this->file( 'path' ) ) && is_writable( dirname( $this->file( 'path' ) ) ) ) ) {

            if ( ! $wp_filesystem->put_contents( $this->file( 'path' ), $content, FS_CHMOD_FILE ) ) {

                // Fail!
                return false;

            }

        }

    }


    /*
     * Determines if the CSS file is writable.
     */
    public function can_write() {

        global $blog_id;

        // Get the upload directory for this site.
        $upload_dir = wp_upload_dir();
        // If this is a multisite installation, append the blogid to the filename
        $blog_id = ( is_multisite() && $blog_id > 1 ) ? '_blog-' . $blog_id : null;

        $file_name   = '/dynamic-css' . $blog_id . '.css';
        $folder_path = $upload_dir['basedir'] . DIRECTORY_SEPARATOR . 'dynamic-styles';

        // Does the folder exist?
        if ( file_exists( $folder_path ) ) {
            // Folder exists, but is the folder writable?
            if ( ! is_writable( $folder_path ) ) {
                // Folder is not writable.
                // Does the file exist?
                if ( ! file_exists( $folder_path . $file_name ) ) {
                    // File does not exist, therefore it can't be created
                    // since the parent folder is not writable.
                    return false;
                } else {
                    // File exists, but is it writable?
                    if ( ! is_writable( $folder_path . $file_name ) ) {
                        // Nope, it's not writable.
                        return false;
                    }
                }
            } else {
                // The folder is writable.
                // Does the file exist?
                if ( file_exists( $folder_path . $file_name ) ) {
                    // File exists.
                    // Is it writable?
                    if ( ! is_writable( $folder_path . $file_name ) ) {
                        // Nope, it's not writable
                        return false;
                    }
                }
            }
        } else {
            // Can we create the folder?
            // returns true if yes and false if not.
            return wp_mkdir_p( $folder_path );
        }

        // all is well!
        return true;

    }


    /*
     * Gets the css path or url to the stylesheet
     *
     * @var     string  path/url
     *
     */
    public function file( $target = 'path' ) {

        global $blog_id;

        // Get the upload directory for this site.
        $upload_dir = wp_upload_dir();
        // If this is a multisite installation, append the blogid to the filename
        $blog_id = ( is_multisite() && $blog_id > 1 ) ? '_blog-' . $blog_id : null;

        $file_name   = 'dynamic-css' . $blog_id . '.css';
        $folder_path = $upload_dir['basedir'] . DIRECTORY_SEPARATOR . 'dynamic-styles';

        // The complete path to the file.
        $file_path = $folder_path . DIRECTORY_SEPARATOR . $file_name;
        // Get the URL directory of the stylesheet
        $css_uri_folder = $upload_dir['baseurl'];

        $css_uri = trailingslashit( $css_uri_folder ) . 'dynamic-styles/' . $file_name;

        // Take care of domain mapping
        if ( defined( 'DOMAIN_MAPPING' ) && DOMAIN_MAPPING ) {
            if ( function_exists( 'domain_mapping_siteurl' ) && function_exists( 'get_original_url' ) ) {
                $mapped_domain   = domain_mapping_siteurl( false );
                $original_domain = get_original_url( 'siteurl' );
                $css_uri = str_replace( $original_domain, $mapped_domain, $css_uri );
            }
        }
        // Strip protocols
        $css_uri = str_replace( 'https://', '//', $css_uri );
        $css_uri = str_replace( 'http://', '//', $css_uri );

        if ( 'path' == $target ) {
            return $file_path;
        } elseif ( 'url' == $target || 'uri' == $target ) {
            $timestamp = ( file_exists( $file_path ) ) ? '?timestamp=' . filemtime( $file_path ) : '';
            return $css_uri . $timestamp;
        }

    }
    
    public function compile_scss() {
        require dirname( __FILE__ ) . '/framework/scss/wp-scss/scssphp/scss.inc.php';
        
        $scss = new scssc();
        $scss->setImportPaths( 'framework/scss/' );
        $scss->setFormatter( 'scss_formatter_compressed' );
        
        $variables = Kirki::get_variables();
        $vars      = '';
        foreach ( $variables as $variable => $value ) {
            $vars .= '$' . $variable . ':' . $value . ';';
        }
        
        $scss_string = $vars . '@import "style.scss";';
        
        $css = $scss->compile( $scss_string );
        
        return $css;

    }

}
$compiler = new Aristath_Dynamic_CSS
// Omit closing PHP tag to avoid "Headers already sent" issues.