A system wide dark theme for stock Android has been long overdue. Meanwhile there have been workarounds such as Substratum that have worked for both root and non-root devices.
A dark mode was in Oreo 8.1.0 developer preview but the feature itself never got into the actual update. Strangely however the wallpaper you use in Oreo can change the notification panel (Quick Settings) into a dark mode.
I discovered this by error after setting up a darkish wallpaper for my home screen. At the time I thought this was a bug but it turns out this is an actual feature built into the system.
|Dark Quick Settings with a Black Wallpaper|
So if you need a dark notification panel just use a wallpaper with dark colours on it. A solid black wallpaper will do the trick if you don’t mind something boring. Otherwise you can try some filters in Photos that may darken your existing wallpaper as I’ve done below:
|Dark Quick Settings with a Filtered Wallapaper|
You however don’t have to use a dark wallpaper for this to work. As a workaround you can install the LWP+ app from the playstore that tricks the OS into thinking you’re using a dark wallpaper while infact you’re using a light one.
Still this may not work on all android devices running Oreo. Works flawlessly on mine but I presume that’s because my phone is running stock android.