Thank you for writing in, your version of X and Cornerstone is not compatible, please update both to the latest version. (X 5.2.5 / CS 2.1.7)
Updating Your Themes and Plugins
Remember to clear all caches (plugin, server-side, CDN, and browser's cache) after updating so that the code from the latest release is always in use. This will help you to avoid any potential errors.
After you update, please reconfigure/resave your font-size settings on Theme Options > Typography.
Hope it helps,