Acceptance criteria

Font toggle — serif / sans-serif Given I am on a reading page, when I switch the font setting, the body text updates immediately to the selected typeface without a page reload.
Colour scheme — light / dark Given I toggle the colour scheme, the background, text, and UI elements switch to the correct palette. Both themes meet WCAG AA contrast ratios.
Text size — S / M / L Given I select a size, the reading area adjusts to the corresponding scale (S = 14px, M = 16px, L = 20px). Line-height scales proportionally to maintain readability.
Settings persist across sessions Given I set my preferences and return later, the same theme, font, and size are applied on load, stored per user account (I hope this is possible)

Basic flow

User click in a document
User clicks the 3 dots contextual option
User clicks: reading preferences.
A floating toolbar/popover anchored near the 3-dot button will appear on top of the page
User click outside of the popover to close it

Why a floating toolbar instead of the right panel?

It's lighter, keeps the reader's eye near the controls, doesn't shift the layout, and feels more native to reading apps. A sliding panel is heavy UX for just 3 settings.

Ideally we will make this consistent when possible with other actions. Versions is one thing makes sense having on a right panel.