custom stylesheets does not apply to optional field title

[lekeCoder]

Version

Tell us which versions you are using:

• tcomb-form-native v0.6.20
• react-native v0.63.4

Expected behaviour

I changed the color of controlLabel normal. it applies to all field titles except optional field

Actual behaviour

Steps to reproduce

My custom stylesheet:

const Form = t.form.Form;
  const formStyles = {
    ...Form.stylesheet,
    formGroup: {
      normal: {
        marginBottom: 10
      },
    },
    controlLabel: {
      normal: {
        color: 'purple',
        fontSize: 18,
        marginBottom: 7,
        fontWeight: '500'
      },
      optional: {
        color: 'purple',
        fontSize: 18,
        marginBottom: 7,
        fontWeight: '500'
      },
      // the style applied when a validation error occours
      error: {
        color: 'red',
        fontSize: 18,
        marginBottom: 7,
        fontWeight: '300'
      }
    }
  }
![Simulator Screen Shot - iPhone 11 - 2020-12-29 at 22 31 34](https://user-images.githubusercontent.com/19432914/103330100-a2c50b80-4a25-11eb-8e3b-12ebf78ecd3a.png)