Use same dark background as web editor

src/window.vala

@@ -157,7 +157,7 @@ public class WriteAs.MainWindow : Gtk.ApplicationWindow {
                 Gtk.rgb_to_hsv(text_color.red, text_color.green, text_color.blue,
                         out h, out s, out v);
 
-                if (v < 0.5) css += "* {background: black; color: white;}";
+                if (v < 0.5) css += "* {background: #222; color: white;}";
             }
             cur_styles = new Gtk.CssProvider();
             cur_styles.load_from_data(css);