Not sure when this happened, but the way Discourse interprets markdown (or maybe markdown itself?) has changed. Not sure if this is a known issue, a deliberate change, something only in night mode for some reason, or what.
Things like # for headings no longer work.
#Is this a heading?
##How about this?
###Not on my screen.
The h1, h2, h3, h4, etc. HTML tags still work fine, though.
Not sure what else has changed, I just noticed it while looking through some of the larger posts that have multiple headings, 'cause they’re all broken now.
EDIT: Google Chrome Version 64.0.3282.186 64-bit, macOS 10.12.6