I have experienced the same problem : the background color of the text / html area is always the same of the one defined in my template_css.css (which is dark in my case).
After searching with other rich text editor (mosCE has the same problem) and testing other templates, here is the solution .
In your template_css.css you must define a css attribute "background" in the body{} element. This color will be the background color of the tinyMCE area.
You must also define a new class element, named ".body_page_bg" for example, where you define a "background" attribute with the bg you want for your layout.
After doing this, in your index.php of your template, modify the <body> tag to add attribute class="body_page_bg"
Example of template_css.css :
Code:
body {
margin: 0px;
background: #FFFFFF;
}
.body_page_bg {
background: #000099;
margin: 0px;
border: 0px;
} index.php
Code:
<body class="body_page_bg">
...
</body>
textarea background in tinymce!!! 
nothing 
Linear Mode
