While it may be possible to add dark status bar icons most apps won't use that feature pre-M using internal version checks. Quick reply is a feature of this module (in the experimental builds, you can find the latest version attached on the last page iirc) though app support is limited, again due to API limitations and version checks.Is was surprised by this beautiful module. I use it with CM12 on S4.
I would only add things (which by the way are implemented by Marshmallow upside down): The dark icons feature on the status bar, on clear app backgrounds. And another thing (which is already implemented on Nougat), the feature of responding to a specific person, in a set of notifications. (Whatsapp, Telegram etc...). The rest is great!