Currently not possible. You can get almost everything in a dark theme, but some windows are unaffected (as in original netbeans) which results in a grand mess like having the structure in gray with dark labels and white text.
People from MHCP said that next version will be 6.00, but they didn't give a timeframe for release (we missed the current release cycle, the next one is in march)
What does this mean? Will they change the base IDE to be the current Apache Netbeans 13 or whatever? That would solve most if not all of the issues related to the old IDE, including this one, but.. who knows!