There are countless threads about dark themes already. An official one is being worked on but we want to make it a lot more comprehensive than just changing some existing colors (you can get themes for that on the forum already), and unfortunately Windows does not make this easy as the dark mode and themes in general in Windows 10 are completely half-baked and unusable, so we have to do everything ourselves. There's no ETA on that, but it's being worked through on and off, and the dark themes on the forum are already pretty good in terms of the main part of the program (i.e. the lister and file displays).
We don't have any plans for an official "dark icon set". The standard icons work fine with dark mode in my opinion. (I've been using them that way for years now.)
There isn't a supported way to extract the internal icons. If you want something styled like the unofficial set that you're using, making your own versions of the missing icons is what I would do, as recoloring icons from a completely different style is unlikely to work very well. You can often cut up the existing icons and join them together in new ways to create the missing parts.