Have you bothered to check through the MyBB themes section?
The first few downloads on the page are some dark themes you can choose from, and you'll find more if you search around.
Here's just a few to start with:
https://nulledbb.com/Thread-RIP-ForumRoot-Ripped-Theme
https://nulledbb.com/Thread-Retail-Ignite
https://nulledbb.com/Thread-RIP-Download...theme-free
https://nulledbb.com/Thread-RIP-Vermilli...s-Powerful
MyBB Requests
MyBB 1.8 Dark Theme
Submitted by panglod, 29-07-2016, 10:22 PM, Thread ID: 22556
Thread Closed