白纸 / 暖黄 / 浅绿 / 灰调 / 夜间 (prototypes 4 and 23) tint only the reading
page — paper, ink, readouts — while the chrome keeps the reading theme.
Stored per device in AppSettings (paper, paperFollowsDark), set from the
reader's 外观 sheet or 外观与主题. 夜间 makes the reader dark; a light paper
gives way to the theme's dark values while the app is dark unless
跟随系统深色模式 is off. Old sepia/green/dark page themes migrate to the
matching paper. Tokens are mirrored in TS and CSS and held to contrast by
theme-tokens.test.ts.
