Just chiming in on the state of cocoa. I have been using svn cocoa trunk for a few months on a few big projects. While I ran into a few glitches, the progress has been fantastic. I've been trying to report bugs, the more people who do, the better. At this point I am no longer planning to use carbon.
lldb is exciting news, it is working well for me.
As far as I can tell, the cocoa fixes are showing up in trunk (2.1), not 2.0, so I am sticking with trunk for Mac probably until 2.2 is stable. I was trying to report cocoa bugs in 2.0.0RC2, but finding them fixed in 2.1 trunk.
On Win and Lin I'll continue to use the latest stable release, which I run on VirtualBox.
I do not have Mojave installed yet, so have no experience with the dark theme.
Cheers,
VTwin