I am having one Gtk+ and C application in which i want to set the font to some XYZ font type and font color to blue, for all the labels and text in the application how can i do
It is generally considered bad form to do that; apps should respect the user's theme. That said, you can deploy a resource file with the settings you want to override in your application, and then use gtk_rc_parse() to load the file at runtime.
Check out GTK+ Resource Files documentation for details.