This is the code that does that:
$assetsUrl = $modx->getOption('np.assets_url', null, MODX_ASSETS_URL . 'components/newspublisher/');
$modx->regClientCss($assetsUrl . 'css/button.css');
Since you don't have an np.assets_url System Setting, the code is using the MODX_ASSETS_URL, which should be https unless you've hard-coded a non-https URL in config.inc.php. I'm surprised this doesn't cause problems elsewhere.
Do you have code to force https in your .htaccess file? That might solve it.
If none of that helps, you can create a System Setting with the key: np.assets_url and put your absolute URL to the assets directory as the value. Be sure it ends in a slash. That should survive any upgrades to NP or MODX.