Changing the font, size or style of a Value Label in a Data Chart Slice or Bar is not obvious. In order to change the font it is necessary to first select the Series box in the Legend corresponding to the Slice or Bar that has the percentage value you wish to change. A white circle will appear in the center of the Series box indicating that it is selected. At this point you can choose any font, size or style.
NOTE: It is necessary to change each slice in a Pie chart individually.