X is the theme. Cornerstone is the page builder. You can use cornerstone a standalone page builder if you bought a license for it. If you have X theme license, it comes with Cornerstone as a bundled plugin. If you will use X theme, it has 4 stacks. Those 4 stacks has predefined styles. Each can be adjusted to your preferred design. Default stack is integrity. To see each stack default design, see this link: https://theme.co/x/demos/. That current content design will adjust on the specific task you will choose. Choosing X theme means you can alter entire site design. What you have right now, it's content and how it looks will adjust on the chosen stack. If you will choose Cornerstone, since it is a page builder, you can use it to create new pages. All other content will still be the same.
Hope this helps.