It's been a long process, but the changes we announced back in March are done. Most sites will see their new themes live for all users on November 30. Some sites with blocking issues may have to wait until December 7. This deadline is important because the new design will make it easier than before to roll out a certain annual festivity. It's been a massive project to design, build and test the changes on 174 sites. Fortunately, it's also served the purpose of dramatically reducing lines of code:
(This was just the final pull request. Additional lines of code were removed as individual site themes were built. I'm also aware that lines of code are but a proxy for code complexity.)
We appreciate your patience with this process and your feedback. Looking back, it was not the best strategy to tackle the most complicated sites first. The earliest sites endured the bulk of the bugs and design issues. Some communities are particularly frustrated with the changes and we appreciate your constructive criticism. I'm gratified that so many people have taken the time to provide us with detailed analysis of design features.
We'll continue fixing bugs and starting in 2019 we'll be working on designs for sites with the stock theme. This process used to take a month or more of designer time. With a standard theme, the process will take far less time so more sites will get customizations sooner. Designers will also think a lot less about CSS details and a lot more about how the final design will look. Sites that have been waiting the longest for their design (Anime and Code Golf come to mind) will be contacted earliest next year on their meta sites.
We're also working on features to make use of the new design. Notably custom question lists will use the left sidebar to display custom queries and indicate whether there are new questions. This should make it much easier to keep track of activity in tags that you care about and would not be possible without the uniform navigation changes just completed.
We can also start looking at usability concerns such as choice of font stack and accessibility problems. There are also pages that aren't yet fully responsive, which need to be fixed. (By the way, the "Disable Responsiveness" button won't be taken away until we've finished that work.)
Does that mean my request was ignored?
We got a lot of feedback. Some issues have been fixed based on that feedback and others have not. We've tried to address all the showstoppers and acknowledge any concerns we don't plan to address. But in between are a lot of great suggestions that we've put off until after the theme has been turned on everywhere. We'll be reexamining that in-between feedback as design work continues. For bigger changes to your site's theme, have a discussion on the site's child meta.
It occurs to me that we've not communicated that this is intended to be an iterative process. Before the standard theme, sites were necessarily locked into their designs permanently. Any change made would be a complete one-off. Now our designers can make changes to a single library that will be available to all Stack Exchange sites, all Stack Overflow Teams and all Enterprise instances. We're not done with site design; we're just getting started.