export const fontSize = (value: number) => {
    const valueStr = value + 'px';
    return {
        fontSize: valueStr
    }
};

export const warning = style(
    fontSize(15), {
        $nest: {
            '& span': ( fontSize(12), {
              backgroundColor: 'yellow'
            })
        }
    });


<div className={warning}>
    This text is formatted correctly
    <span>this text is not</span>
</div>