Editor background colour in full screen

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.

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; 
}