Reported by @lookitsnicholas on Discord...

[...] what happened to the black title bar on macOS [...] there was a dark grey/black one in RC8

... and from Apple:

"The system automatically opts in any app linked against the macOS 10.14 or later SDK to both light and dark appearances."

So any builds against older SDKs -- like our stable-1.2 build, built on macOS 10.13 -- will use the light theme. RC8 was running a custom build built on 10.14 and that's why it was dark before.

Desired vs. Actual