Hi,
As far as I know the only bold you will see is the name of the notebook when it is not synchronized. By default the workbook you are working in is displayed at the top of the notebook pane. This is not bold faced displayed (at least not in my OneNote 2013)

My notebooks are not synchronised & yet the one I am working in is not bold. Besides, according to what I have read, irrespective of it being synchronised or not, the title active notebook (i.e. the one one is currently working in) in the left hand pane should always be in bold.