Reduced hard coded stylesheet values

* disabled hardcoded wizard pages style sheet
* adjusted style sheet
* fixed QMneu issue: https://stackoverflow.com/questions/53991839/qt-5-12-menu-items-with-unwanted-extra-space
* tagged remaining ones as "\fixme hardcoded"

src/blackgui/stylesheetutility.cpp

@@ -156,7 +156,7 @@ namespace BlackGui
                 const QString f = fileInfo.fileName().toLower();
 
                 // keep even empty files as placeholders
-                newStyleSheets.insert(f, c);
+                newStyleSheets.insert(f, c); // set an empty string here to disable all stylesheet
             }
             file.close();
         }