For those like me using Trellis, here is how it seems to me is the best-est way to do it.
Since it’s a configuration you’ll probably want on all your environments, thus edit ./site/config/application.php
Around line 39 you should see:
define('CONTENT_DIR', '/app');
define('WP_CONTENT_DIR', $webroot_dir . CONTENT_DIR);
define('WP_CONTENT_URL', WP_HOME . CONTENT_DIR);
Add a line right underneath:
define('WP_DEFAULT_THEME', CONTENT_DIR . '/themes');
Done.