How can I achieve an internal border effect on windows? (pictures)

[liiyung]

Referring to the effect in the attached picture + other screenshots ("magical sunset", "cherry blossom") here – is it currently possible to achieve the internal border effect?

If yes – how would one go about achieving it? Hopefully I can add it in myself if I understand what to do.
If no – could I make this a feature request?

Thank you!

[koekeishiya]

Unfortunately this is not really something that is possible to do on macOS, sorry.