There is no single place where you can find the default CSS code for a Nabble application. It is spread over a number of CSS files. The easiest approach is probably to examine the source code of the Nabble page in your browser and work from there.
In any case, you don't really don't need to know the default. Just write the appropriate code to over ride the default into the dialogue reached from the CSS button on the toolbar revealed after navigating: OPTIONS > APPLICATION > CHANGE APPEARANCE.
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