Okay, so lets clear up a couple of things here.
1. Dark Mode has been available since 10.10, it's a developer choice. I've written about it a couple of times in xDevMag and my own Retina Kit will help you get there. http://ohanaware.com/retinakit/ To see some examples.
2. For icons, you don't colorize them yourself. You supply the system a grayscale image with alpha and it will colorize them to match. My early tests with Mojav, show not only the Dark Interface being quite different to the one before, but some of the colors are different. So if you colorize yourself, you're going to have to use different constants, especially as some of the colors are 'magic' colors and cannot be correctly rasterized into Xojo colors.
3. For a while I've been working on Retina Kit 4, trying to improve how it works with Xojo, over the next few weeks I am adapting the newer code into a "macOS Theme Kit", which will make it a lot easier for Xojo developers to 'theme' their apps using as much system control as possible, i.e. to be the most compatible.
4. Try using ICNS mini to compress macOS icons. https://itunes.apple.com/us/app/icnsmini-shrink-png-icns-iconsets-tifs-icos/id1035260885?mt=12 I will have a 64-Bit version ready later on this year.
5. The version of the macOS Theme Kit I'm working on already features support for system colors, in theory you use these colors to build custom interfaces. However they do NOT match actual system controls in any interface, some of them are close and yet some are so far away it's almost comical that Apple expects 3rd party developers to use these to make custom controls, knowing that it means custom controls will never be exact matches.