input { background-color : red }
input:indeterminate { background-color : lime}

<form>
<input type="radio" name="rg" value="1"></input>the background of this checkbox
          should be green<br></br>
<input type="radio" name="rg" checked="checked" value="2"></input>the background of this checkbox
          should be green<br></br>
<input type="radio" name="rg" value="3"></input>the background of this checkbox
          should be green<br></br>
</form>