Declaring a nested CSS class in JSS takes the following form:

import {createUseStyles} from 'react-jss';

const styles = createUseStyles(() => ({
  primaryCSSClass: {
    color: 'black',
    paddingLeft: '50px',
    '& .nestedCSSClass': {
        color: 'red',
    }
  }
}));

In your JSX you would apply the CSS classes as follows:

const MyComponent = () => {

const classes = useStyles();

    return (
        <div className={classes.primaryCSSClass}>
            <p>Some text</p>
            <div className="nestedCSSClass">
                <p>Another text</p>
            </div>
        </div>
    );
};