Thanks for reaching out.
The required plugin for X theme is just cornerstone and it's automatically installed once you visit Admin > X > Validation. But of course, you'll wait a bit for it to be installed
There are bundled plugins to the theme but they are optional depending on what you need to achieve. Example, if you wish to display a slider then you must install Revolution Slider plugin. You can find the bundled plugins on the same area of Admin > X > Validation.
But you have to validate it first to allow bundled plugin installation.