Hey @vlad,
The file you need is feature-box.php and that’s located in \wp-content\themes\pro\cornerstone\includes\shortcodes.
Since it’s not recommended to edit the core files directly and there’s no way to override the elements, I’d recommend you create an element of your own based on the Classic Featured List element.
You can learn how to create elements in our Themeco Docs specifically under the Developer Resources.
Please just note that we do not provide custom development support here as part of product support.
If you have custom development questions, you might want to avail of One where we answer questions beyond what’s offered in themes and plugins.
Hope that helps.