What do you mean saying "this"? What is the code you use to change the font-family? The code I've provided already works fine and does the job (bg color and font color).

@anonymous3542 I solved all of this problems by install plugin "TinyMCE Advanced".

Look at the screenshot.

@robert - essentially, it's this. However, the usual declaration for a font outside of the system installed is usually encased in quotes so I wonder if these need to be escaped 

<script type="text/javascript">
    jQuery(window).load(function() {
        tinyMCE.activeEditor.dom.addStyle('body{background-color:#000000 !important; color:#dddddd !important; font-family:lato !important;}');
    });
</script>

I've also seen some circumstances where font-family needs to be fontFamily or similar in TinyMCE .

The single font doesn't work for you. Try to change it to this:

font-family: Lato, Helvetica, sans-serif !important;