Out of curiosity, does anybody know why the default IntelliJ dark theme was changed from Darcula to the new Dark theme? Just for variety? Or is that a result of some sort of UX research?

I often find myself questioning why it's not set to "system default" by default, as is the case with most other apps. This is usually the first setting I adjust because I prefer the interface to be light during the day and switch to a dark mode in the evening.

I agree. It ought to be system default. If you like the dark theme, you would set it as your system default. If you don't, you would keep the light theme as your system default. Why change it in multiple places? (While I'm at it, I have the same annoyance for many other things too that disrespect consistency, for example the font sizes on medium.com - if I wanted large fonts I would change them in my browser settings for all sites.)