Since my previous post I have removed the hack and reverted back to the original NC color.For anyone trying to use Foxlet's hack, you may have to completely disable System Integrity Protection (SIP) before you even use the Terminal to turn off Notification Center in step 1.
And it's likely that Notifications aren't going to work (any notifications already in NotificationCenter will disappear and no new ones will show up)—and some of the ControlCenter Widgets that involve interaction from you (like calculator) won't work, though some appear to work fine.
I've tried this hack on 4 different machines (all running HighSierra 10.13.3 beta) and while they all get the dark appearance, none of them actually work with this hack—though they all fail in different ways. Perhaps it's because I'm running the beta on all the machines.
While it originally looked good there were several things that did not work properly, so for me at least it was a no-go.