The Discussion board is yet another List View so you can apply custom XSL to it. This is maybe the recomended SharePoint like way, but dealing with XSL is time consuming and if your XSL file has even one error in it, the SharePoint would reject it and reverse the dafault list view so this costs patience and precision.
One you have completed your custom XSL you can apply it by accessing the webpart properties (in Edit mode) -> Miscellaneous -> XSL Link (where you specify where your XSL file is located on the server)