input { background:red }
 input:not(:indeterminate):not(:checked) { background:green; }

 <div>

  <script>
   

    function test() {
      document.getElementById("test").checked = false;
    }

    window.setTimeout("test()", 100);
   
  </script>

  <p> <input id="test" type="checkbox" checked="checked"> This checkbox should have a green background. </p>

 </div>