How to Conditionally set the Value of a Field Based on the Value of Another Field

{
  "colorTheme": "forest",
  "textColor": "#fefefe",
  "backgroundColor": "#179e40"
}

{
  name: 'theme',
  title: 'Theme',
  type: 'object',
  inputComponent: ThemeSelect,
  options: {
     themes: [{ name: 'forest', textColor: '#fefefe', backgroundColor: '#179e40' }]
  }
  fields: [
    {
      name: 'textColor',
      title: 'Text color',      
      type: 'string'
    },
    {
      name: 'backgroundColor',
      title: 'Background color',      
      type: 'string'
    }
  ]
}