One warm dark theme instead of two
The built-in Dark theme has been retired. It had drifted to within a few percent of Ember (same warm charcoal canvas, same amber-orange accent, differing mainly in how dark the window frame sits), and two themes that close apart are a choice nobody can make meaningfully. Ember stays: near-black chrome across the title bar, tab strip, sidebar, and status bar, a warm charcoal workspace, and the brighter ember accent. Light, Midnight, and Sepia are unchanged, so the picker in Settings → Appearance now offers four built-in themes rather than five.
If you were using Dark, the app moves you to Ember on the next launch rather than dropping you into a light theme, and remembers it from then on. Custom themes are untouched. Ones you built by duplicating Dark keep every token you saved, still render exactly as before, and their Reset to base button now restores Ember's tokens. Behind the scenes the dark token set still exists as the fallback layer for any custom dark theme, so a custom theme that never defined a token continues to inherit a sensible dark value for it.