Fonts weren't loading because the paginator renders inside a sandboxed
iframe. @font-face declarations in the parent page's CSS are invisible
to the iframe's document. Even injecting @font-face rules via
setStyles() may not trigger font loading in sandboxed iframes.
Fix: inject a <link> to reader-fonts.css directly into the iframe's
document on each section load, so @font-face declarations are parsed
in the iframe's own document context where font-family rules can
reference them.
Also:
- Remove foliate-themes.css entirely (no longer needed)
- Set viewport background color directly via JS using THEME_COLORS map
- Remove reading theme CSS classes from viewport element