fix: improve card separation and overlay contrast

app/lib/themes/card_overlay_colors.dart

@@ -0,0 +1,26 @@
+import 'package:flutter/material.dart';
+
+class CardOverlayColors {
+  CardOverlayColors._();
+
+  static const background = Color(0x99000000);
+
+  static Color selection(ColorScheme colors) =>
+      _foreground(colors.brightness == Brightness.light ? colors.primaryContainer : colors.primary);
+
+  static Color favorite(ColorScheme colors) =>
+      _foreground(colors.brightness == Brightness.light ? colors.errorContainer : colors.error);
+
+  static Color _foreground(Color preferred) {
+    // A white cover is the brightest possible backing behind the dark scrim.
+    // High-contrast and e-ink schemes can supply dark container colors.
+    final brightestBackground = Color.alphaBlend(background, Colors.white);
+    final contrast = (preferred.computeLuminance() + 0.05) / (brightestBackground.computeLuminance() + 0.05);
+
+    if (contrast >= 3) {
+      return preferred;
+    }
+
+    return Colors.white;
+  }
+}

app/lib/widgets/library/acquisition_placeholder_card.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 import 'package:papyrus/acquisition/acquisition_models.dart';
 import 'package:papyrus/themes/design_tokens.dart';
+import 'package:papyrus/themes/card_overlay_colors.dart';
 import 'package:papyrus/widgets/library/acquisition_status_text.dart';
 import 'package:papyrus/themes/app_motion.dart';
 import 'package:papyrus/widgets/shared/app_progress_indicator.dart';
@@ -187,7 +188,7 @@ class _SelectionIconButton extends StatelessWidget {
     final colorScheme = Theme.of(context).colorScheme;
 
     return Material(
-      color: Colors.black45,
+      color: CardOverlayColors.background,
       borderRadius: BorderRadius.circular(AppRadius.full),
       child: InkWell(
         borderRadius: BorderRadius.circular(AppRadius.full),
@@ -197,7 +198,7 @@ class _SelectionIconButton extends StatelessWidget {
           child: Icon(
             selected ? Icons.check_circle : Icons.radio_button_unchecked,
             size: IconSizes.small,
-            color: selected ? colorScheme.primary : Colors.white,
+            color: selected ? CardOverlayColors.selection(colorScheme) : Colors.white,
           ),
         ),
       ),

app/lib/widgets/library/book_card.dart

@@ -4,6 +4,7 @@ import 'package:papyrus/models/book.dart';
 import 'package:papyrus/providers/book_storage_status_controller.dart';
 import 'package:papyrus/providers/enums/library_reading_status.dart';
 import 'package:papyrus/themes/design_tokens.dart';
+import 'package:papyrus/themes/card_overlay_colors.dart';
 import 'package:papyrus/utils/book_actions.dart';
 import 'package:papyrus/widgets/book/private_book_cover.dart';
 import 'package:papyrus/widgets/library/acquisition_status_text.dart';
@@ -108,6 +109,7 @@ class _BookCardState extends State<BookCard> {
           margin: EdgeInsets.zero,
           clipBehavior: Clip.antiAlias,
           color: isUnavailable ? colorScheme.surfaceContainerLow : null,
+          shadowColor: colorScheme.brightness == Brightness.dark ? colorScheme.outline : null,
           child: InkWell(
             onTap: effectiveTap,
             child: Column(
@@ -140,7 +142,7 @@ class _BookCardState extends State<BookCard> {
                             child: _CardIconButton(
                               tooltip: widget.isFavorite ? 'Remove from favorites' : 'Add to favorites',
                               icon: widget.isFavorite ? Icons.favorite : Icons.favorite_border,
-                              color: widget.isFavorite ? colorScheme.error : Colors.white,
+                              color: widget.isFavorite ? CardOverlayColors.favorite(colorScheme) : Colors.white,
                               onTap: widget.onToggleFavorite != null
                                   ? () => widget.onToggleFavorite!(widget.isFavorite)
                                   : null,
@@ -157,7 +159,7 @@ class _BookCardState extends State<BookCard> {
                                 ? 'Deselect ${widget.book.title}'
                                 : 'Select ${widget.book.title}',
                             icon: widget.isSelected ? Icons.check_circle : Icons.radio_button_unchecked,
-                            color: widget.isSelected ? colorScheme.primary : Colors.white,
+                            color: widget.isSelected ? CardOverlayColors.selection(colorScheme) : Colors.white,
                             onTap: widget.onSelectToggle,
                           ),
                         ),
@@ -466,7 +468,7 @@ class _CardIconButton extends StatelessWidget {
         container: true,
         button: true,
         child: Material(
-          color: Colors.black45,
+          color: CardOverlayColors.background,
           borderRadius: BorderRadius.circular(AppRadius.full),
           child: InkWell(
             borderRadius: BorderRadius.circular(AppRadius.full),

app/lib/widgets/shelves/shelf_card.dart

@@ -59,6 +59,7 @@ class _ShelfCardState extends State<ShelfCard> {
       margin: EdgeInsets.zero,
       clipBehavior: Clip.antiAlias,
       elevation: AppElevation.level1,
+      shadowColor: colorScheme.brightness == Brightness.dark ? colorScheme.outline : null,
       child: InkWell(
         onTap: widget.onTap,
         onLongPress: widget.onLongPress,

app/test/widgets/library/card_overlay_contrast_test.dart

@@ -0,0 +1,73 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:papyrus/acquisition/acquisition_models.dart';
+import 'package:papyrus/themes/app_theme.dart';
+import 'package:papyrus/widgets/library/acquisition_placeholder_card.dart';
+import 'package:papyrus/widgets/library/book_card.dart';
+
+import '../../helpers/test_helpers.dart';
+
+void main() {
+  final themes = {
+    'light': AppTheme.light,
+    'dark': AppTheme.dark,
+    'high contrast light': AppTheme.highContrastLight,
+    'high contrast dark': AppTheme.highContrastDark,
+    'e-ink': AppTheme.eink,
+  };
+
+  final job = AcquisitionJob.fromJson({
+    'job_id': 'contrast-job',
+    'title': 'Downloading book',
+    'status': 'queued',
+  });
+
+  final cards = <String, Widget>{
+    'favorite': BookCard(book: buildTestBook(), isFavorite: true),
+    'selected book': BookCard(
+      book: buildTestBook(),
+      isFavorite: false,
+      isSelectionMode: true,
+      isSelected: true,
+    ),
+    'selected download': AcquisitionPlaceholderCard(job: job, isSelectionMode: true, isSelected: true),
+    'unselected book': BookCard(book: buildTestBook(), isFavorite: false, isSelectionMode: true),
+    'unselected download': AcquisitionPlaceholderCard(job: job, isSelectionMode: true),
+  };
+
+  for (final theme in themes.entries) {
+    for (final card in cards.entries) {
+      testWidgets('${card.key} stays visible over cover colors in ${theme.key}', (tester) async {
+        await tester.pumpWidget(
+          MaterialApp(
+            theme: theme.value,
+            home: Scaffold(
+              body: SizedBox(width: 200, height: 300, child: card.value),
+            ),
+          ),
+        );
+
+        final icon = switch (card.key) {
+          'favorite' => Icons.favorite,
+          'selected book' || 'selected download' => Icons.check_circle,
+          _ => Icons.radio_button_unchecked,
+        };
+
+        final finder = find.byIcon(icon);
+        final foreground = tester.widget<Icon>(finder).color!;
+        final material = tester.widget<Material>(find.ancestor(of: finder, matching: find.byType(Material)).first);
+
+        for (final cover in [Colors.white, Colors.black, Colors.red, Colors.green, Colors.blue]) {
+          final background = Color.alphaBlend(material.color!, cover);
+          final foregroundLuminance = foreground.computeLuminance();
+          final backgroundLuminance = background.computeLuminance();
+          final lighter = foregroundLuminance > backgroundLuminance ? foregroundLuminance : backgroundLuminance;
+          final darker = foregroundLuminance < backgroundLuminance ? foregroundLuminance : backgroundLuminance;
+          expect((lighter + 0.05) / (darker + 0.05), greaterThanOrEqualTo(3), reason: 'Cover: $cover');
+        }
+
+        expect(tester.takeException(), isNull);
+      });
+    }
+  }
+}