baseStyle.js

const styles = theme => ({
  container: {},
  input: {},
})

export default styles

ComponentA.js

import React from 'react'
import baseStyle from './baseStyle'

const styles = theme => ({
  ...baseStyle,
  bar: {
    width: theme.spacing.unit * 3
  }
})

const ComponentA = ({ classes }) => <div className={classes.container}></div>

export default withStyles(styles)(ComponentA)

ComponentB.js

import React from 'react'
import baseStyle from './baseStyle'

const styles = theme => ({
  ...baseStyle,
  foo: {
    height: theme.spacing.unit * 6
  }
})

const ComponentB = ({ classes }) => <div className={classes.container}></div>

export default withStyles(styles)(ComponentB)