As far as I am aware, changing the look and feel through the menu will only affect the bits that use Java Swing for the user interface… which is the case for some elements of ImageJ (e.g. the Command Finder), but not everything. The changes are more evident if you switch to ‘Modern mode’ in Fiji.
However, even then there aren’t a great variety of different ‘default’ look and feels for Swing, and creating a new one doesn’t look like the easiest task; there is some info at https://docs.oracle.com/javase/tutorial/uiswing/lookandfeel/index.html
On the other hand, for Java user interfaces created with JavaFX then it can be much easier to change the appearance and create a dark theme. This would involve using CSS rather than whatever Swing is relying on.
Switching the ImageJ user interface to use JavaFX would be a rather large task, although @cmongis has already done a huge amount in that direction. I don’t know the current status of ImageJ FX, but it does look very nice indeed.