[NTG-context] Piecewise margin line

[Johann Birnick]

Hello,

I want to setup some fancy theorem style for my math notes. For this I want to
have a colored margin line next to my theorems. It should look like this:

https://i.ibb.co/F6L59pD/idea.png

The line should stretch over the whole theorem, not just the first line.
(But the latter would be a beginning, though.)

Important to note is that the line should be *on the margin*, not in the text.

So do you have an idea on how I could do that? I would greatly appreciate your
answer. Thank you!

Best regards,
Johann