Well, this is embarrassing. While trying to fix an annoying problem with my existing design for this site, I accidentally broke the design, forcing me to grab what I thought were backup copies of the files for that design… only to find out that they were for the previous design. Which is why the site has reverted to its 2012 look.
On the bright side, at least this design isn’t broken. I think.
Update: I found the correct backup files and have re-uploaded the stylesheets. Whew!.