Please don't duplicate the questions. 

Just follow this support topic: https://wpforo.com/community/how-to-and-troubleshooting-2/changing-background-color-of-tinymce-editor/#post-46384

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

Look at the screenshot.

Hello. Could you give me some pointers into how you configured the TinyMCE Advanced plugin to change the editor background color? I have been beating my head against the walls for hours now and cannot seem to make it work. I would really appreciate your help. Thanks. Eric

Sofy is right, there is no way to affect the editor style using CSS code. Try to use JS code in footer.php file of your WordPress active theme. Find the footer.php file and put this code after <?php wp_footer() ?> function. The black bold codes are color codes (#000000 is the background, #dddddd is the font color), you can change those before using:

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

It should look like this:

https://wpforo.com/wp-content/uploads/2018/03/wpForo-change-tinyMCE-background-color.png

Unfortunately, this change only works on some of the editor backgrounds. It does not work on the post edit editor, as well as a couple of others. Any ideas about those?

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

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

@robert Thanks, this works perfectly.

Much appreciated.

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

<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>

Look at the screenshot.