I don't think your phone has the option to activate the dark mode. I have a p20 and it doesn't have the option either. The only way is by downloading this app https://play.google.com/store/apps/details?id=systems.maju.darkmode
It's Google feed. It turned black after I used an application on Google play store called "Dark Mode" . You can find it in the following link:
try lawnchair 2 its amazing or nova launcher you can customize almost everything the only issue i have is the animation in gesture navigation (but thats a huawei issue) besides that there is no difference
I am using an app called "Greenify" for that. It is really closing the apps. But you cannot select system apps, like Google Chrome, just what you install manually...
I have explained to many people that the phone manager doesn't really close the apps, but they don't believe until I gave them Greenify. But be careful, once you choose to close Messenger or whatever used for Instant Messaging, it will be KILLED FOR REAL, meaning that you will not receive notification until relaunched.
Their interface is very intuitive. Just follow the steps.
EDIT: Here is the link
https://play.google.com/store/apps/details?id=com.oasisfeng.greenify