Add design spec for Reddit fonts override feature

Co-authored-by: Cursor <cursoragent@cursor.com>

docs/superpowers/specs/2026-08-27-reddit-fonts-design.md

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+# Reddit Fonts Override
+
+Replace old Reddit's default fonts with Reddit's modern typeface family (Reddit Sans, Reddit Sans Condensed, Reddit Mono) from the [reddit/redditsans](https://github.com/reddit/redditsans) repository.
+
+## Scope
+
+- Full page override — all text on old.reddit.com and www.reddit.com.
+- Toggleable from the extension popup (one switch for all three families).
+- Font files bundled in the extension (no external network requests).
+
+## Font Assets
+
+Three variable-weight woff2 files stored in `content/fonts/`:
+
+| File | Family | Approx Size |
+|---|---|---|
+| `RedditSans[wght].woff2` | Reddit Sans | 96KB |
+| `RedditSansCondensed[wght].woff2` | Reddit Sans Condensed | 94KB |
+| `RedditMono[wght].woff2` | Reddit Mono | 81KB |
+
+Variable fonts only — each covers all weights (Light through ExtraBold) via the `wght` axis. No italic variants initially; can be added later if needed.
+
+Licensed under SIL Open Font License 1.1.
+
+## CSS Architecture
+
+New file: `content/styles/fonts.css`, registered in `manifest.json` in the `css` array.
+
+### @font-face Declarations
+
+Three `@font-face` blocks, one per family. Extension-relative paths (e.g., `url("../fonts/RedditSans[wght].woff2")`). Each declares `font-display: swap` and the full `font-weight` range (300–800).
+
+### Override Rules
+
+All overrides gated on `html.rt-fonts-enabled`:
+
+- `html.rt-fonts-enabled` sets `font-family: 'Reddit Sans', sans-serif` on the root.
+- Body, content, sidebar, footer inherit from root.
+- `code`, `pre`, and `.md code` elements use `font-family: 'Reddit Mono', monospace`.
+- Reddit Sans Condensed is declared but not force-applied — available as `'Reddit Sans Condensed'` for future tweaks or user stylesheets.
+
+## Settings & Toggle
+
+### Default
+
+`fontOverride: true` added to `DEFAULTS` in `lib/settings.js`. On by default.
+
+### Class Toggle
+
+`content/main.js` toggles `html.rt-fonts-enabled` on init and on settings change, following the same pattern as `rt-card-enabled` and `rt-badges-enabled`.
+
+### Popup
+
+New checkbox "Reddit fonts" in the top section of `popup/popup.html`, alongside "Enabled" and "Badge layout". Wired in `popup/popup.js` the same way as existing toggles.
+
+No per-family granularity — one switch controls all three families on or off.
+
+## Files Changed
+
+| File | Change |
+|---|---|
+| `content/fonts/RedditSans[wght].woff2` | New — bundled font asset |
+| `content/fonts/RedditSansCondensed[wght].woff2` | New — bundled font asset |
+| `content/fonts/RedditMono[wght].woff2` | New — bundled font asset |
+| `content/styles/fonts.css` | New — @font-face declarations and override rules |
+| `manifest.json` | Add `content/styles/fonts.css` to `css` array |
+| `lib/settings.js` | Add `fontOverride: true` to `DEFAULTS` |
+| `content/main.js` | Toggle `rt-fonts-enabled` class |
+| `popup/popup.html` | Add "Reddit fonts" checkbox |
+| `popup/popup.js` | Wire the new checkbox |