The classes prop created by the withStyles function is meant to be an optional parameter where you can provide some or all of the style classes.

Imagine a somewhat contrived snippet like:

import React from "react";
import withStyles, { WithStylesProps } from "react-jss";
import clsx from "clsx";

const styles = {
  root: { display: "block" },
  blue: { color: "blue" },
};

type Props = { blue?: boolean } & WithStylesProps<typeof styles>;
const Component = ({ blue = false, classes }: Props) => (
  <div className={clsx(classes.root, { [classes.blue]: blue })}>
    Hello, world!
  </div>
);

const StyledComponent = withStyles(styles)(Component);

You'd expect to be able to use it in any of the following ways:

<StyledComponent />
<StyledComponent blue />
<StyledComponent classes={{ root: "myClass" }} />

The last use case was broken in TypeScript because, while classes was an optional prop, each key within class was required. So you'd get an error like:

Property 'blue' is missing in type '{ root: string; }' but required in type 'Record<"blue" | "root", string>'.ts(2741)

This changes makes sure that the classes property is still optional, but that each key within it is optional, too. I abstracted a field out so that I could use it in two places. There's probably about a dozen ways to set something like this up, though, so if you'd prefer it formatted in a different way let me know.