Going by the screen shot, it looks like you are using AppStyling, probably by loading an ISL file. You can edit these files in AppStylist, although you might find that application to be somewhat cumbersome as it is targeted toward visual designers. What I usually do is just open it up in notepad and search for the role name and UI state you want to change. In this case, the role in question is probably either GridCell or GridRow, and the state is 'Selected', so you can search the file for those strings. You'll know you found it when you see the bold attribute; just remove that and the text will no longer be bolded.

If you have any trouble with this, repost with the ISL attached and I'll do it for you.