DarkPanel is a PanelContainer-base variation (only a `panel`
stylebox); a Button looks up normal/hover/pressed, which don't exist
on it, so SystemDock's menu buttons rendered as default gray Godot
buttons instead of mock-faithful dark nav buttons.
Add a dedicated Button-base DockButton variation (Palette-only
colours, dock-dark normal/hover/pressed styleboxes), point
system_dock.gd's menu buttons at it, and regenerate the committed
game_theme.tres artifact from the builder.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
