I don't know about non X-windows applications, but by default, *any* Tk program on X-windows is "skinnable". It's called the options database, and for a user, can be configured using .Xdefaults, or, runtime, with any other file and the xrdb program.

You could of course write your own API and configure the widgets accordingly.

As for you question about no titlebars, etc, call the overrideredirect(1) method in your toplevel widgets.