In the first developer preview for Android M, which was released during Google I/O earlier this year, there was a dark theme included in the preview. However it was a bit hidden in the developer settings, and definitely not ready for primetime. It was actually removed in the second preview, and is still missing in the third preview. Many users loved that Google, at least it seemed, was going to give us the option of having a dark or a light theme throughout the OS. But according to a Googler on the bug tracker for AOSP, it appears that the dark theme will not be in the final release for Marshmallow which is due for later this month. However, he did say it could be in a future release. Now whether that's a future Marshmallow release, or Android N next year or even later on, is anyone's guess.
Customization has been a big feature for Android since its inception about 8 years ago now. Being able to switch out the launcher for something else, swapping icon packs and so much more. It's a huge selling point for many people, and having a system-wide dark theme is something that Xposed has actually done for a good little while now. So many of us were excited to see a dark mode in Android M. Although we are a bit disappointed that it is now being removed, at least for the time being. Hopefully we won't have to wait to much longer to see the dark option make its way back into vanilla Android.
Android 6.0 Marshmallow is due to be released pretty soon. With the rumor of new Nexus devices coming on September 29th, it's a big possibility that we will see the final Marshmallow build that day or shortly after. While Marshmallow doesn't look like a huge update, underneath the UI, there are a ton of new features, as well as bug fixes. Which still warrant the update to version 6.0 from 5.1.1. As usual, it will be available first on the new Nexus devices from LG and Huawei and likely coming to the Nexus 6, Nexus 9, Nexus 5 and Nexus 7 2013 afterwards.