Defer body inline style removal until CSS variables are injected by
useApplyTheme, preventing a one-frame flash of unstyled background.
Replace preloader logo <img> with CSS mask technique matching the React
DittoLogo component, so the logo uses the theme's primary color instead
of filter:invert() which turned purple into green.
Read the user's theme from localStorage before first paint so the
preloader background and spinner colors match dark/light/black/pink.
Uses a small blocking script (public/theme.js) that runs before
the preloader is visible.