[bbuehne]

Ok, found it. If you edit:

editor_contents.css

(user CP - Styles and Templates - style manager - chose edit template options from dropdown box next to go button. Double-click 'CSS templates' in left box, then 'editor_contents.css)

The code controlling the text and background in the WYSIWYG editor is in this section at the very top:

body,
body.content {
font: {vb:stylevar content_msg_font};
color: #ffffff;
/* Remove the background color to make it transparent */
/*background-color: #fff;*/
background-color: #25292C;
margin:0px;
padding:8px;
}

Color controls font color and back-ground color of the editor. I've edited my text to be white at the background the gunmetal blue of the forums. The old code in those lines was:

color: {vb:stylevar postbit_color}
background-color: {vb:stylevar wysiwyg_textbox_background.backgroundColor}