You’re talking around 350 images that would need variants, and so around 175 code switches for that (since all images have two variants, Retina and non-Retina). Then every background element such as header bars, footer bars, inspector bars, split view dividers and so on would need alternative drawing code. Anything using colour such as labels would need alternative ways of drawing to work with dark backgrounds. Custom button selections would need alternative drawing routines. Apple elements with no dark variant, such as outline and table header bars, would need overriding and custom drawing applied. And so on…
It took the best part of a month to implement Dark Mode in iOS, which has a fraction of the UI elements the macOS version has. So you can imagine how that would play out for macOS!
A more feasible approach would be to offer an option for using a darker grey for headers, footers and inspector backgrounds that still works with current colours and elements, and then to allow theme Preferences to do the rest.