fix(theme): dark adw-gtk3 for dark palettes — GTK dialog buttons (#98)

Stylix always used the light adw-gtk3 sheet with recolored tokens; that
sheet hardcodes near-black button labels, which became black-on-dark in
Boreal GTK dialogs. Force adw-gtk3-dark (and prefer-dark) for dark mode,
and pin dialog button label colours in Stylix extraCss.

agent/BACKLOG.md

@@ -31,22 +31,6 @@ post-boot sessions). Preserve separation: the installer bake failure and the
 flake pin are different root causes even when they show up on the same machine.
 (Terminal / Ghostty-on-Acer → shipped as Kitty-only, #95.)
 
-### 98. Boreal: button text renders black and is difficult to read
-
-Audit GTK button foreground/background contrast under Boreal, including the
-installer surface where it was observed. Follow THEME-DESIGN's two-theme
-visual protocol. Pass = normal, hover, focused, and disabled button labels
-remain legible and palette-consistent.
-
-**Progress 2026-07-13:** scripted checks pass all 24 themes — not
-palette-level. Stylix gtk.css for Boreal audited: fg roles are light
-(#d3dae0); the near-black values are `accent/warning/error_fg_color`
-(#21272f) on their pastel bgs (deliberate, ~4.4:1). Candidate surfaces:
-adw-gtk3 (light base) + dark recolor edge cases, or gum's TUI confirm
-buttons in the installer (VM gum screens under Boreal look legible —
-artifacts in `/tmp/nomarchy-v2-swap-93/`). Needs Bernardo's screenshot or
-an Acer repro to pin the actual widget before fixing.
-
 ### 127. `[blocked:hw]` Docked idle: black screens, no wake, undock does not restore panel
 
 Bernardo 2026-07-15 (AMD dev box, docked): left the laptop docked; after a