Current, up-to-date tutorial documentation for Ruby Tk is very sparse, 
especially for newer features like the themed widgets.  I've been 
working on a website that I hope helps fill some of those gaps.  You can 
find it here:

      http://www.tkdocs.com

Please check it out, and in particular the tutorial, which covers the 
more modern "best practices" with using Tk.  It gives equal coverage to 
using Tk both from Tcl and from Ruby (you can turn either off of 
course), so you don't have to try to translate from Tcl-oriented 
documentation to figure out how to do things in Ruby.

I hope some people here will find this helpful.  Any corrections, 
contributions or comments definitely appreciated!

Thanks
Mark