Address PR review:
- Add theme.cfg ([Style] Name = Fusion, ColorScheme = System) to Fabric,
Leather, Plasma and VelvetMarble so their curated palettes actually apply.
Without it the native style paints button chrome from the OS (windowsvista
on Windows has no dark mode), making the dark palettes' light ButtonText
unreadable on light buttons.
- Leverage the scheme-variant asset resolution: register home-dark.png and
home-light.png in resources and add them to the qrc so the built-in home
background also flips with the palette instead of staying static.
- Fix Leather [Palette.Inactive] Accent, which copied the Active Highlight
color instead of the theme accent (dark #4a5f8f -> #c9995a, light
#34508c -> #a5712f) in both palette files.
