The change was intentional. The goal was to make the text stand out more and to make secondary UI elements blend a little better.
The idea is that pretty much all the space in the composer that’s not taken by buttons/shorter input elements is the text area. Or in the case of the default desktop composer it’s split into two columns, for writing and for preview, so even a slight visual separator could be enough.
In any case, I’d like to give this change some time, and after an acclimation period we could adjust the colors. Maybe split the difference between the original border color and the current one. We’ll see!
Some of the fields didn’t have their border color adjusted. Fixed in a follow-up PR. Same with the iOS margin.