Sidebar and Queue painted their own opaque bg-bg, which sat in front of
the fixed, full-viewport app-backdrop div (z-index: -10) that carries
--mf-ambient's radial-gradient glow in dark mode. Since the main content
area has no background of its own, the glow only ever showed through
there — sidebar and queue blocked it with a flat, gradient-less fill of
the same base color.
Drop bg-bg from both so the same app-backdrop (color + ambient gradient)
shows through underneath them too. In light mode this is a no-op
(--mf-ambient is 'none' and app-backdrop's flat color already matches
what bg-bg painted), so only dark mode is visually affected.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>