Add this code to your wp-config.php file when installing a self-contained WordPress install on this site. This will ensure that the themes and plugins directories pull from the main wp-content directory on this server. The additional code will ensure that the uploads are left within the individual site install.
define( 'WP_CONTENT_DIR', '/var/www/marketing/wp-content' );
define( 'WP_CONTENT_URL', 'https://marketing.mdpweb.net/wp-content' );
define( 'UPLOADS', 'wp-content/uploads' );
// define( 'WP_PLUGIN_DIR', '/var/www/marketing/wp-content/plugins' );
// define( 'WP_PLUGIN_URL', 'https://marketing.mdpweb.net/wp-content/plugins');
// define( 'PLUGINDIR', '/var/www/marketing/wp-content/plugins' );