feat: add create_style function with light/dark theme support

docs/superpowers/specs/2026-05-13-epub-reader-design.md

@@ -0,0 +1,162 @@
+# ePub Reader — Design Document
+
+## Overview
+
+A lightweight ePub reader desktop application built with Rust + egui, compiled as a single Windows executable (x86_64-pc-windows-gnu / MSYS2+MinGW). No console window. Built-in Chinese font support. Kindle-like reading experience with medium feature set.
+
+## Tech Stack
+
+| Layer | Choice |
+|---|---|
+| GUI Framework | `eframe` (egui official framework) |
+| Rendering Backend | `wgpu` |
+| ePub Parsing | `epub-rs` |
+| Font | Bundled Noto Sans SC (or similar open-source Chinese font) |
+| Build Target | `x86_64-pc-windows-gnu` |
+| Binary | Single `.exe`, `#![windows_subsystem = "windows"]` |
+
+## Core Data Model
+
+```rust
+struct AppState {
+    book: Option<Book>,
+    current_section: usize,
+    current_page: usize,
+    settings: Settings,
+    bookmarks: Vec<Bookmark>,
+    sidebar_open: bool,
+}
+
+struct Book {
+    title: String,
+    author: String,
+    cover: Option<Vec<u8>>,
+    sections: Vec<Section>,
+    toc: Vec<TocEntry>,
+}
+
+struct Section {
+    title: String,
+    content: String,           // plain text (HTML tags stripped)
+    pages: Vec<usize>,         // character offsets for page boundaries
+}
+
+struct Settings {
+    font_size: f32,            // default 20.0
+    theme: Theme,
+}
+
+enum Theme { Light, Dark }
+
+struct Bookmark {
+    section: usize,
+    page: usize,
+    label: String,
+    timestamp: i64,
+}
+
+struct TocEntry {
+    label: String,
+    section: usize,
+    children: Vec<TocEntry>,
+}
+```
+
+## Architecture
+
+```
+┌──────────────────────────────────────────┐
+│              eframe::App                  │
+│  ┌──────────┐  ┌───────────────────────┐  │
+│  │ WelcomePg │  │    ReadingView        │  │
+│  │ (no book) │  │  ├─ TopToolbar        │  │
+│  │ ────────  │  │  ├─ Sidebar (TOC)    │  │
+│  │ open btn  │  │  ├─ TextArea          │  │
+│  │ recents   │  │  └─ BottomProgressBar │  │
+│  └──────────┘  └───────────────────────┘  │
+│  ┌──────────────────────────────────────┐ │
+│  │           AppState                    │ │
+│  └──────────────────────────────────────┘ │
+└──────────────────────────────────────────┘
+```
+
+## UI Layout
+
+### Welcome Screen (no book open)
+- Centered "Open ePub File" button
+- Recently opened books list (read from settings file)
+
+### Reading Screen (Kindle-like)
+- **Top toolbar**: back button, book title, font size controls, menu, bookmark toggle, theme toggle
+- **Left sidebar** (collapsible): table of contents tree
+- **Center**: text content area with pagination
+  - Click left 30% → prev page, right 30% → next page, center 40% → toggle toolbar visibility
+  - Keyboard: ← → for prev/next page
+- **Bottom bar**: current chapter name, progress slider, page number indicator
+
+## Features (Medium Scope)
+
+### Core
+1. Open `.epub` file via native file dialog
+2. Parse ePub (OPF manifest, NCX/TOC, Spine)
+3. Render chapter content with pagination
+4. Next/previous page navigation (click & keyboard)
+5. Table of contents sidebar with chapter navigation
+6. Remember last reading position (bookmark per file)
+
+### Settings & Display
+7. Font size adjustment (A⁺/A⁻)
+8. Light/Dark theme toggle
+9. Chinese font support (bundled Noto Sans SC)
+
+### Bookmarks
+10. Add/remove bookmarks at current page
+11. Bookmark list view
+
+### Persistence
+12. `settings.json` saves: font_size, theme, recent files, reading positions, bookmarks
+
+## Data Flow
+
+1. User clicks "Open" → native file dialog → select `.epub`
+2. `EpubLoader` reads zip, extracts OPF metadata, reads spine order, parses NCX for TOC
+3. For each spine item: extract HTML content, strip tags → plain text stored in `Section.content`
+4. On section load: calculate pagination based on current font_size + panel width → store page offsets
+5. On page change: slice `Section.content[pages[current_page]..pages[current_page+1]]` → render to egui `Label`
+6. On font_size change: recalculate all pagination for current section
+7. On theme toggle: switch `egui::Style` between light/dark visuals
+
+## Pagination Algorithm
+
+```
+fn paginate(text: &str, font_size: f32, panel_width: f32) -> Vec<usize>
+  chars_per_line = floor(panel_width / (font_size * 0.6))
+  lines_per_page = floor(panel_height / (font_size * 1.5))
+  chars_per_page = chars_per_line * lines_per_page
+  Iterate text, split at chars_per_page boundaries (prefer word/char boundaries)
+```
+
+## File Storage
+
+- `settings.json` (next to exe OR in standard config dir)
+  - font_size, theme
+  - recent_files: Vec<{path, title}>
+  - reading_positions: HashMap<path, {section, page}>
+  - bookmarks: HashMap<path, Vec<Bookmark>>
+  - Last window size/position
+
+## Error Handling
+
+- ePub parsing failure → show error dialog with message
+- File not found (recent list) → remove from list silently
+- Font loading failure → fall back to system monospace
+- Settings file corruption → reset to defaults, log warning
+
+## Out of Scope (v1)
+
+- Text search within book
+- Highlighting / annotations
+- Notes/export
+- TTS
+- Epub with complex CSS styling / reflowable layout math
+- DRM-protected ePubs

src/theme.rs

@@ -1,3 +1,5 @@
+use eframe::egui;
+use eframe::egui::Style;
 use serde::{Deserialize, Serialize};
 
 #[derive(Debug, Clone, Copy, PartialEq, Serialize, Deserialize)]
@@ -12,6 +14,19 @@ impl Default for Theme {
     }
 }
 
+pub fn create_style(theme: &Theme) -> Style {
+    match theme {
+        Theme::Light => Style {
+            visuals: egui::Visuals::light(),
+            ..Default::default()
+        },
+        Theme::Dark => Style {
+            visuals: egui::Visuals::dark(),
+            ..Default::default()
+        },
+    }
+}
+
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct Settings {
     pub font_size: f32,
@@ -69,6 +84,18 @@ mod tests {
         assert_eq!(restored.theme, s.theme);
     }
 
+    #[test]
+    fn test_create_style_light_vs_dark() {
+        let light = create_style(&Theme::Light);
+        let dark = create_style(&Theme::Dark);
+        // Light and dark should have different window fills
+        assert_ne!(light.visuals.window_fill, dark.visuals.window_fill);
+        // Dark mode should have darker window
+        assert!(dark.visuals.window_fill.r() < light.visuals.window_fill.r());
+        assert!(dark.visuals.window_fill.g() < light.visuals.window_fill.g());
+        assert!(dark.visuals.window_fill.b() < light.visuals.window_fill.b());
+    }
+
     #[test]
     fn test_theme_serialize() {
         let json = serde_json::to_string(&Theme::Dark).unwrap();