It's a free custom theme for Windows, downloaded from Deviant Art. It's called Matte, and it affects the entire Windows look, Notepad, with some third party apps too. For example, ArchiCAD and SketchUp get affected. Internet browsers, Blender, GIMP, Adobe apps - don't. Office bundle - kind of. It vastly decreases contrast in ArchiCAD GUI, and some buttons are barely visible, but I like it anyway. I also set a darker background for my projects, and it seriously saved my eyes when working late at night. There are many custom themes available to download. Some of them are free, many are like $2-3. Steam theme looks nice too.
There are things though that you need to keep in mind, if you want to install them. You'll find instructions at the download pages and you better read them carefully before you hack your system, so it allows installing third party themes. All in all, it's worth it if you like dark GUIs. I can't live without them. I also recommend Dark Reader addon if you use Firefox.
See my post there:
https://archicad-talk.graphisoft.com/viewtopic.php?f=24&t=66173&p=301337#p301337
That's how it looked back when I used Steam theme.
Matte theme you can download from there:
https://www.deviantart.com/kdr3w/art/Matte-758699852
ArchiCAD 25
Windows 10