Your Quick Reply currently has a black (#2C2C2C) background color and the text is white (#FFFFFF). That is set here in additional.css:
HTML Code:
.wysiwyg,.formcontrols .blockrow .editor_textbox textarea:focus,.editor_textbox textarea,.editor_textbox iframe,body.wysiwyg{color:#fff;background-color:#2c2c2c;
although, note that you forgot the closing } above. That means the rest of your CSS in the additional.css file is not valid and won't show up.