login_screen.js

var styles = require('styles');

// creates { color: 'blue', font: { fontSize: 16 }, textAlign: 'right' }
var usernameLabel = Ti.UI.createLabel(_.defaults({
  textAlign: 'right'
}, styles.style_classes.Label, styles.style_ids.username_label));

styles.js

exports.style_classes = {
  'Window': {
    backgroundColor: 'white'
  },
  'Label': {
    color: 'blue'
  }
};

exports.style_ids: {
  'login_screen': {
    'username_label': {
      font: {
        fontSize: 16
      }
    }
  }
};