How to extends classes in material UI createStyles

import { createStyles, Theme } from '@material-ui/core/styles';

export default (theme: Theme) => {
    const { primary } = theme.palette;

    return createStyles({
        test1: {
            fontSize: '30px',
        },
        test2: {
            '& > div': {
                color: primary.main,
                marginTop: '20px',
            },
        },
    });
};

Here I want to use class test1 in test2. How should I extend it?