The warnings are caused by this line in your demo.js file:
<Items {...this.props} items={items} />
You’re spreading all of List‘s props down into your Items. One of these props is classes, containing all of the CSS classes you define in demo.js. Since those are intended for List, they include CSS classes that are implemented by List but not Items. Since Items is receiving this prop, it’s reading it as you trying to override classes that aren’t available and warning you about it.
You can fix this problem by spreading only the unused props:
// Use other to capture only the props you're not using in List
const { classes, headerIsHidden, ...other } = this.props;
// Then spread only those unused props
<Items {...other} items={items} /
Then, you won’t be spreading classes object into Items, so you won’t get any warnings about classes that aren’t implemented.