I noticed that on the classic Battle.net, the screen is stretched a bit on the monitor. Any ways to fix it? It would be more pleasant to play and for viewers.

Check if your video card offers an aspect ratio scaling option; that works for me with Nvidia.

What did you change it to?

Edit: I got it Thanks!

To play the GOG version of Diablo 1 in original 4X3 aspect ratio (on Windows 10 with a 1080p monitor).

1. Use the version in the ...\GOG\Diablo\dl folder
In this folder is another copy of Diablo 1 - but, with some Microsoft DirectX stuff

2. Execute ...\GOG\Diablo\dx\dxcfg.exe and select 1280x1024 Display Mode

3. Set Presentation = Windowed (Fullscreen does not work)

4. Select Save

5. Set the Windows 10 Taskbar settings to "Automatically hide taskbar in desktop mode" = "on"

6. Double click on Diablo.exe to launch the game.
Do not double-click on DiabloLauncher.exe to start the game

Oddly, if you run:
...\GOG\Diablo\dx\dxcfg.exe
and change to 1920x1080
This gives a 4:3 aspect ratio in full-screen-mode.

If you play around with ...\GOG\Diablo\dx\dxcfg.exe you will
find it provides Diablo in various 4:3 aspect displays in various ways.

Thanks man! worked perfect!