Accessing previous theme variables in createMuiTheme

You'd need to create an instance of the default theme and use it when defining your own:

import { createMuiTheme } from 'material-ui/styles';

const defaultTheme = createMuiTheme();

const theme = createMuiTheme({
  typography: {
    fontSize: defaultTheme.typography.fontSize + 2
  }
});

export default theme;

You can also create your theme and then add on to it after theme is created.

import { createMuiTheme } from 'material-ui/styles';

const theme = createMuiTheme();
theme.typography = {
  ...theme.typography,
  fontSize: theme.typography.fontSize + 2
}

export default theme;