Use always black as the foreground color in HelpViewer

Our css styling assumes black as the background color.
Task-number: QTCREATORBUG-13660
Change-Id: Ia8d76c39c970b436a0ae038191d8d0f8f8728ded
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

src/plugins/help/qtwebkithelpviewer.cpp

@@ -404,6 +404,7 @@ QtWebKitHelpViewer::QtWebKitHelpViewer(qreal zoom, QWidget *parent)
 
     QPalette p = palette();
     p.setColor(QPalette::Base, Qt::white);
+    p.setColor(QPalette::Text, Qt::black);
     setPalette(p);
 
     connect(m_webView, SIGNAL(urlChanged(QUrl)), this, SIGNAL(sourceChanged(QUrl)));