Since D-RATS is a GTK application, it does not obey system-wide theme settings on platforms where GTK is not the native or primary toolkit (i.e., Windows and MacOS). To change the colors of various widgets and the fonts used, you need to edit the gtkrc file manually. Note that this will affect all GTK applications on your system (which is probably what you want anyway). See this link on the pidgin site for information on where to find this file and how to edit it to suit your needs.