While working on JDK-8289334 we experimented with dark theme for javadoc generated documentation. The results can be viewed here:

http://cr.openjdk.java.net/~hannesw/dark-mode/

We are not currently planning to integrate CSS themes in documentation generated by JavaDoc, but the standard stylesheet provides an easy way to implement themes via custom stylesheet. Since the way to do this is not widely known we should provide a guide on how to create and apply CSS themes for JavaDoc-generated documentation.