docs(book): add public Theming & appearance page

docs/src/SUMMARY.md

@@ -7,6 +7,7 @@
 - [Installation](./installation.md)
 - [First run & configuration](./first-run.md)
 - [Downloading](./downloading.md)
+- [Theming & appearance](./theming.md)
 - [Staying ahead of YouTube's bot detection](./anti-bot.md)
 - [Troubleshooting](./troubleshooting.md)
 

docs/src/theming.md

@@ -0,0 +1,51 @@
+# Theming & appearance
+
+catacomb's desktop UI is built on [egui](https://github.com/emilk/egui) and
+ships with **nineteen themes** spanning four moods, plus a three-mode video
+list layout you can toggle on the fly.
+
+## Themes
+
+Pick a theme from **Settings → Theme**. It persists across restarts.
+
+| Category | Themes | Vibe |
+|---|---|---|
+| **Catacomb** (default) | Dark, Light | The out-of-box experience — fully tuned, not stock egui. |
+| **Goth** | Dracula, Vampire, Witching Hour, Cemetery Moss, Emo: Nocturnal, Emo: Coffin, Emo: Scene Queen | Macabre, regal, or neon-goth. Where the app's name lives. |
+| **Neon / retro** | Cyberpunk, Synthwave '84, Vaporwave | Magenta-on-black, sunset gradients, pastel aesthetic. |
+| **Dev palettes** | Nord, Gruvbox, Tokyo Night | The beloved, legible, community-tested schemes. |
+| **Cozy (light)** | Paper, Honey, Candlelight | Warm, low-glare counterpoints to the dark themes. |
+| **Trans** | Trans | Trans flag colours — light blue, pink, white. |
+
+Every theme drives the full surface: panel fills, widget states (hovered,
+active, open), selection rings, hyperlink and accent colours, and the
+warning/error tones. The selection, "now playing", and bulk-selection rings
+all derive from the active theme's accents — so in *Witching Hour* a selected
+video glows arcane-violet, in *Honey* it glows amber.
+
+## Video list layouts
+
+The video list has three render modes, switchable live from a 3-segment
+toggle in the list header:
+
+- **List ☰** — dense horizontal rows (thumbnail left, title + metadata
+  right). Best for large libraries; the default.
+- **Card ▢** — the same horizontal layout, but each row is a rounded card
+  with a hover lift. A more modern, contained feel without losing density.
+- **Grid ◫** — YouTube/Plex-style vertical cards (thumbnail on top, title
+  and metadata below), with the column count adapting to the window width.
+  Best for browsing by eye.
+
+The toggle is **global by default, overridable per view**. Set your preferred
+mode once in **Settings** and it applies everywhere — but if you want, say,
+Grid for *All Videos* and List for a specific channel, flip the toggle while
+in that view and the override is remembered separately. A view with no
+override falls back to the global default.
+
+Card density (thumbnail size and spacing) is separately adjustable via the
+existing density slider in Settings, and applies to all three modes.
+
+## Placeholders
+
+Videos without a downloaded thumbnail show a consistent placeholder — a
+theme-tinted gradient with a subtle glyph — rather than a bare grey box.