Customize Readium layout via --RS__ CSS vars, not by forking
Customizing the visual layout of a Readium-based ebook/audiobook reader (Storyteller, Thorium, custom apps, etc.) when the JS preferences API doesn't expose the knob you need
Readium-CSS uses CSS custom properties prefixed with --RS__ for nearly every paginated layout knob — --RS__colGap, --RS__colCount, --RS__pageGutter, --RS__lineHeight, --RS__baseFontSize, etc. The JS-side EpubPreferences interface in @readium/navigator only surfaces a subset (e.g. columnCount yes, columnGap no), so the instinct when missing one is to fork Readium. But the entire Readium stylesheet references these via var(), so overriding any --RS__* on the iframe contentDocument is enough — Readium reads it the same as its own defaults. Storyteller has a function called applyThemeToDocument that already accepts a document arg and gets called for both the parent and the iframe (in preferencesListeners.ts), so adding a new injection like ["--RS__colGap", ${preferences.columnGap}px] is a 1-line wire-up. To find the right var name for a given layout property, grep the upstream readium/readium-css repo (the modules subdir splits by concern: ReadiumCSS-pagination.css for layout, ReadiumCSS-fs_normalize.css for type, etc.).
When a Readium-based reader doesn't expose the preference you need, grep readium/readium-css for --RS__ vars matching the property, then inject the var on the iframe contentDocument via your reader's existing applyTheme/setCss path. Much simpler than touching @readium/navigator or Readium itself.