"Dark Frame" theme: Highlight the active empty document

The current active and empty document should be highlighted. That was
broken with the "Dark Frame" theme.

Change-Id: I0632f654c146492b7e7a9d61db31e2d85c3d52fd
Reviewed-by: Alessandro Portale <alessandro.portale@theqtcompany.com>

share/qtcreator/themes/darkframe.creatortheme

@@ -41,7 +41,7 @@ DoubleTabWidget2ndSeparatorColor=ffff0000
 DoubleTabWidget2ndTabActiveTextColor=ffffffff
 DoubleTabWidget2ndTabBackgroundColor=ffff0000
 DoubleTabWidget2ndTabInactiveTextColor=ff000000
-EditorPlaceholderColor=normalBackground
+EditorPlaceholderColor=ffdddddd
 FancyToolBarSeparatorColor=43ffffff
 FancyTabBarBackgroundColor=shadowBackground
 FancyTabWidgetDisabledSelectedTextColor=toolBarItemDisabled