Indeed as part of our commitment with our users we decided to give support to HiDPI displays. To do that, and among other things, we had to redesign all our icons since they were not ready for HiDPI at all. HiDPI needs two different sizes per icon, for example the toolbar now needs 64x64 icons in addition to the usual 32x32. We only had the low resolution ones so we had to look for a solution.
All our icons were made by profesional designers, one after the other in the last two decades. At the beginning the idea was to find similar icons and we actually did so. However the result was quite unsatisfactory because it really seems color icons are something of the past, especially in the main toolbar. Lets have a quick look at Apple Safari, iTunes, Mail, Preview, Photos etc. They all use black and white icons. And since MS Windows and macOS seems to copy each other we decided to go for it and replace all those color icons with black and white's.
We already had those B/W icons, in lots of different resolutions, almost ready to be used. We just needed to adjust the size and the color a bit, we actually decided to use dark and light grey icons in a size similar to Apple iTunes. As a result needless to say that we have saved lots of time and lots of money.
I am fully aware that it is a radical turnover and despite people are avid of new versions of our software they dislike big changes, as I do. But by forgetting what we had before and using the new look for a few days you really get used to it. After releasing the new versions I got very few long time users complaining. I can testify most people are happy with it (or maybe they simply don't care).