So the answer is to copy the CSS file from /wp-content/plugins/dw-question-answer/assets/css/tinymce.css to /wp-content/themes/theme-slug/css/tinymce.css and add a CSS rule to it.
eg. body#tinymce{background-color: #333; color: #EEE; font-size:14px;}
…and then use the dwqa_editor_style filter in theme functions.php to point to it, eg.

add_filter('dwqa_editor_style','dwqa_custom_tinymce_css'); 
function dwqa_custom_tinymce_css() {return get_stylesheet_directory_uri().'/css/tinymce.css';}