for what it's worth, Notepad++ is highly configurable. Here's what mine looks like with borders set to 0 and some sidebar items disabled. Even though it is the same classic theme, it looks a lot better than default.

There's the more substantial point to note that Notepad++ is not interactive, the need to have an extremely tight visual coupling between the margin and the main body on a line-per-line basis is significantly reduced. Likewise, the line numbers separated by a border in the IDLE editor are not really bothersome either, it looks natural enough. It only really sticks out in the interactive interpreter.