Directory Opus is developed by (or used) which tool?

I would like to know, for the GUI part, Directory Opus is developed by which tool? Pure WinAPI, MFC, ATL, or it used Delphi to develope?
I asked this question since I found the GUI interface is really elegant. I would like to make a pdf viewer, SumatraPDF, with more GUI support. This means with a GUI which support tabs, dual panel and preview.
Hence I would like to have a GUI interface like Directory Opus.

The GUI is all Win32, C++ without any frameworks, etc.

Thanks Leo, it's amazing that the elegant GUI is developed in Win32 without framework!

No wonder it's so snappy!