I managed to fix this by loading my own css styles (mystyles.css), I think the problem is caused becauce WYSIWYG in heading styles sets the font weight to normal.
If you want me to document this in more detail I happy to do so.
In styles, you cannot select a 'thin' font and at the same time set bold to false. This is not correct.
In your style the 'bold' property should be empty.