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answered at

This kind of problems are solved using Chrome Inspector or Firefox Firebug.

Inspecting the element, we get the necessary ID's or Classes to add in the stylesheet.

chrome inspector

So, .mceContentBody.wp-fullscreen-editor is the one we need to address in the theme file editor-style.css:

.mceContentBody.wp-editor, .mceContentBody.wp-fullscreen-editor {     
  background-color: #000;
  color: #fff; 
}

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