Native std-widgets (LineEdit/TextEdit) render from Slint's own Palette, which
we never synced to the QBZ theme, so on a light theme their text collided with
its own background (Settings input fields, effectively invisible). Add
Theme.is-dark (computed in Rust from the surface luminance, so it is correct
for static/auto/custom themes) and drive Palette.color-scheme from it in the
AppWindow: light/dark follow the theme, System stays `unknown` (follows the
OS). Reactive on theme change.
cc
vicrodh
8df8c18ab0