For about a month and a half now, GTK 3.20 has come in Debian testing. While this is good as GNOME 3.20 was released about couple of months back it broke the themes that are in Debian. A bug has been filed for that .
There has been little in way of documentation about how to fix the themes which reminds me of how Pulseaudio was off-putting when it first came to most people due to incomplete documentation, the breakage and not knowing where to look to fix things or even look up unit-tests, functional tests to understand where the breakage was happening.
It does seem that the GNOME team dropped the ball on this release. A more interesting read of the issue can be seen in this bug thread . I especially like the answer that Michael gave which does tell how much undocumented it really is.
Although if you read Mathias MClasen’s blog post it seems more or less in-line with how regular CSS web-development takes place, so pretty much in-line with GNOME’s stated goals of making GNOME very much a web-development platform so more web-developers could write and support GNOME rather than having to learn C or some other language to write for it. This has been their one of the stated goals from long back, I think circa 2012 or even before.