Use light/dark class instead of colors

[ppo]

Add options custom(Light|Dark)Class to add a CSS class instead of defining color styles on the element.

let contrast = new Contrast({
  customLightClass: 'contrast-el-light',
  customDarkClass: 'contrast-el-dark',
  // …
});

So in your example:

<h1 class="color-block contrast-el">Resize and watch my color change</h1>

On dark backgrounds, the element would become:

<h1 class="color-block contrast-el contrast-el-light">Resize and watch my color change</h1>