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

 <div>

  <script>
   

    function test() {
      /* Microsoft specs for .indeterminate say that this is an invalid test
         as an element can be both .checked and ,indeterminate; however the
         selectors spec says no element can be :checked:indeterminate. */
      document.getElementById("test").indeterminate = true;
      document.getElementById("test").checked = true;
    }

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

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

 </div>