MainApplication: Load private.css style from theme in private browsing

Closes #2575

src/lib/app/mainapplication.cpp

@@ -1016,6 +1016,10 @@ void MainApplication::loadTheme(const QString &name)
         qss.append(QzTools::readAllFileContents(activeThemePath + QLatin1String("/rtl.css")));
     }
 
+    if (isPrivate()) {
+        qss.append(QzTools::readAllFileContents(activeThemePath + QLatin1String("/private.css")));
+    }
+
     qss.append(QzTools::readAllFileContents(DataPaths::currentProfilePath() + QL1S("/userChrome.css")));
 
     QString relativePath = QDir::current().relativeFilePath(activeThemePath);