STR:
Settings -> Preferences -> General -> Localization: select a RTL language (e.g. Hebrew).
Restart NPP.
Preferences -> Dark Mode.

Result:

Notepad++ v8.3.2 (64-bit)
Build time : Feb 27 2022 - 16:11:01
Path : C:---\Notepad++ Original.exe
Command Line :
Admin mode : ON
Local Conf mode : ON
Cloud Config : OFF
OS Name : Windows 10 Pro (64-bit)
OS Version : 2009
OS Build : 19042.985
Current ANSI codepage : 1255
Plugins : ComparePlus.dll HTMLTag.dll MenuIcons.dll PythonScript.dll _CustomizeToolbar.dll