Notepad++ changed in the past couple of years, and it requires a few extra steps to set up a dark theme.
The answer by Amit-IO is good, but the example theme that is needed has stopped being maintained. The DraculaTheme is active. Just download the XML and put it in a themes folder. You may need Admin access in Windows.
C:\Users\YOUR_USER\AppData\Roaming\Notepad++\themes