Epub reader themes (#97)

* Implemented a custom theme engine for the EPUB reader, allowing users to switch between built-in themes (System, Light, Dark, Sepia, Slate, and OLED).

Key changes include:
- Added `ReaderThemePanel` and theme selection logic to `EpubReaderScreen`.
- Introduced a theme persistence layer using SharedPreferences.
- Updated `ChapterWebView` and `epub_reader.js` to dynamically apply background and text colors via JavaScript, including improved contrast adjustment for inline styles.
- Enhanced `CssParser` and `ContentStyler` in the paginated reader to adapt CSS colors based on the active theme's background and text luminance.
- Integrated theme-specific background and text colors into `PaginatedReaderScreen` and its layout components.
- Added a theme palette icon to the reader controls.

* Added custom theme builder and texture support to EPUB reader

* Refactored color picker(PDF) into shared components and enhanced EPUB theme editor

* Updated theme customization UI and improved WebView theme application efficiency

* Updated page flip animation to use paper color with a theme-based tint

* Improved state restoration during paginated reader reconfiguration

app/src/main/java/com/aryan/reader/epubreader/EpubReaderControls.kt

@@ -152,6 +152,7 @@ fun EpubReaderTopBar(
     onOpenTtsSettings: () -> Unit,
     onOpenDeviceVoiceSettings: () -> Unit,
     onOpenDictionarySettings: () -> Unit,
+    onOpenThemeSettings: () -> Unit,
     searchFocusRequester: androidx.compose.ui.focus.FocusRequester,
     modifier: Modifier = Modifier,
     onToggleReflow: (() -> Unit)? = null,
@@ -209,6 +210,13 @@ fun EpubReaderTopBar(
                             contentDescription = "Dictionary Settings"
                         )
                     }
+                    TooltipIconButton(
+                        text = "Theme",
+                        description = "Theme Settings",
+                        onClick = onOpenThemeSettings
+                    ) {
+                        Icon(painter = painterResource(id = R.drawable.palette), contentDescription = "Theme Settings")
+                    }
                     Box {
                         var showMoreMenu by remember { mutableStateOf(false) }
                         TooltipIconButton(