Thanks for asking.
You are using kind of the latest version of Cornerstone 3.1.5. However, version of X Theme 6.0.4 is old and not compatible with Cornerstone version you are using. In that regards I suggest you to update X Theme and Cornerstone to most recent release which as of today is X Theme 6.1.6 and Cornerstone 3.1.6.
You can take a look at the theme update guide for instructions on updating theme: