Hi!

I think that you can't achieve it with just CSS - you need to use SCSS / SASS or a little bit JavaScript, I made a sample solution with second one for you, try this code:

HTML:

<div class="switch">
  <label id="label">
    Off
    <input type="checkbox" id="input" required />
    <span class="lever"></span> On
  </label>
</div>

JS

  const attr = $('#input').attr('required');
  if (attr) {
    $('#label').css('color', 'red');
  };

I hope it'll meet your expectations :)

Best regards