In <RC root>/config/config.inc.php add the following setting:
// Skin name: folder from skins/
$config['skin'] = 'elastic';
You can also set the following setting if you want to disable any skins:
// Limit skins available for the user.
// Note: When not empty, it should include the default skin set in 'skin' option.
$config['skins_allowed'] = [];