Every screen but Login set `backgroundColor: bg` itself, so the login page and
the boot loader fell through to the platform's white surface while the text
tokens stayed light — white on light, unreadable. The palette in
`theme/tokens.ts` is fixed dark by construction, and `Uniwind.setTheme` defaulted
to light, so heroUI's own surfaces were light too. Paint the background at the
root, on the login screen and on the loader, and default the theme to dark.