How do you change the visual editor’s background color?

similar to the answer by GavinR, using the wordpress add_editor_style() function; (as implemented in Twenty Ten and Twenty Eleven)

in functions.php of your theme, add:

add_editor_style();

then create an editor-style.css in your theme folder:

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