In short: no chance. I mean, you can try, but it will be a lot of work.
There are a few obstacles:
- The themes (excluding the basic theme) are only responsible for about 25% of the code. I try not to keep dead code, so most of the other 75% of code is actually needed.
- Webstart has a cache, the users will only need to download the jar files once.
- don’t expect me to help by writing any code or build files, because this “feature” just does not yield enough benefits. This framework was built for power users, and power users usually have no issue downloading 2 mb.
You could however try to obfuscate the code. Right now the class files are full of debug symbols and long class/method/variable-names. Removing or shortening them may yield some bytes. I cannot tell you how much can be gained with obfuscation - and of course you lose any chance of getting readable exceptions.