I'd say that it's an advanced task to learn and master
all the rules required to build a look&feel for Java 1.4.

You may consider reading reading Sun's Swing tutorial
and the articles provided at Sun's Swing connection.
The latter explain the look&feel architecture and can
help you build your first l&fs.

Kunststoff and my Windows l&f and my Plastic l&f family
may serve as examples. On the other hand all these l&fs
contain code tricks, often because Jan and me extend
a Metal or Basic-L&f class and need to work around changing
APIs over different versions.

I suggest that you seriously consider writing a look&feel
that is based on Sun's SynthLookAndFeel. Synth provides
a well-designed API that makes it much easier to build
your custom look. If you want to experiment with Synth,
I recommend to download the Java 1.5 beta. The 1.4 API
requires to write a UI delegate subclass even for tiny
changes in both the appearance and UI gestures handling.

In contrast Synth enables you to describe a look with
fewer classes (for advanced stuff) or even with a bunch
of graphics and an external description (for most stuff).

I'm not aware of Synth-based third-party looks.
You could be one of the first - in the scene or market.
Feel free to use the UI gestalt as provided by Plastic
and feel free to copy the Plastic color themes -
all is open source under the BSD license terms.

Hope this helps. Best regards,
Karsten Lentzsch
JGoodies :: Java User Interface Design