How to use the spectacle/lib/themes/default/screen.js function in spectacle

secondary: '#fafafa',
  tertiary: '#ccc',
  quartenary: '#cc99cd',
  white: '#fafafa',
};

const fonts = {
  heading: "'BioRhyme', serif",
  body: "'Work Sans', sans-serif",
  monospace:
    "'Silkscreen', 'Inconsolata', Consolas, 'Lucida Console', 'Lucida Sans Typewriter', 'DejaVu Sans Mono', 'Bitstream Vera Sans Mono', 'Liberation Mono', Monaco, Courier, monospace",
};

// Default Spectacle Theme for Screens
import screen from 'spectacle/lib/themes/default/screen.js';
const defaultTheme = screen(colors, fonts);

// We're doing it. Let's deep merge the default theme from spectacle
// with our custom theme overrides, without losing any default styles.
// Source: https://davidwalsh.name/javascript-deep-merge
const isObjectWithValue = val => {
  return val && typeof val === 'object';
};

const deepmerge = (target, source) => {
  return mergeObject(target, source);
};

const mergeObject = (target, source) => {
  let dest = {};

  if (isObjectWithValue(target)) {