fix(android): improve dark mode text readability and contrast

- Add dedicated dark mode color scheme with high contrast text
   - Create values-night resources for optimized dark theme
   - Update UI components to use MaterialTheme dynamic colors
   - Replace hardcoded colors with theme-aware colors in Compose UI
   - Fix widget text colors to adapt to system theme
   - Improve task status colors for better visibility in dark mode

android-app/app/src/main/java/com/tunpok/calendarwidget/ui/components/TaskCard.kt

@@ -32,7 +32,8 @@ fun TaskCard(
         modifier = modifier.fillMaxWidth(),
         shape = RoundedCornerShape(12.dp),
         colors = CardDefaults.cardColors(
-            containerColor = Color.White
+            containerColor = MaterialTheme.colorScheme.surface,
+            contentColor = MaterialTheme.colorScheme.onSurface
         ),
         elevation = CardDefaults.cardElevation(
             defaultElevation = 2.dp
@@ -75,13 +76,13 @@ fun TaskCard(
                     Text(
                         text = "间隔: ${task.minIntervalDays} 天",
                         style = MaterialTheme.typography.bodySmall,
-                        color = Color.Gray
+                        color = MaterialTheme.colorScheme.onSurfaceVariant
                     )
                     task.lastExecutionTime?.let { time ->
                         Text(
                             text = " | 上次: ${formatDate(time)}",
                             style = MaterialTheme.typography.bodySmall,
-                            color = Color.Gray
+                            color = MaterialTheme.colorScheme.onSurfaceVariant
                         )
                     }
                 }
@@ -128,7 +129,8 @@ fun TaskDisplayCard(
             .clickable { onClick() },
         shape = RoundedCornerShape(16.dp),
         colors = CardDefaults.cardColors(
-            containerColor = SurfaceColor
+            containerColor = MaterialTheme.colorScheme.surface,
+            contentColor = MaterialTheme.colorScheme.onSurface
         )
     ) {
         Column(

android-app/app/src/main/java/com/tunpok/calendarwidget/ui/control/ControlScreen.kt

@@ -39,17 +39,17 @@ fun ControlScreen(
                     )
                 },
                 colors = TopAppBarDefaults.topAppBarColors(
-                    containerColor = PrimaryColor,
-                    titleContentColor = Color.White
+                    containerColor = MaterialTheme.colorScheme.primary,
+                    titleContentColor = MaterialTheme.colorScheme.onPrimary
                 )
             )
         },
         floatingActionButton = {
             FloatingActionButton(
                 onClick = { viewModel.showAddDialog() },
-                containerColor = PrimaryColor
+                containerColor = MaterialTheme.colorScheme.primary
             ) {
-                Icon(Icons.Default.Add, contentDescription = "添加任务", tint = Color.White)
+                Icon(Icons.Default.Add, contentDescription = "添加任务", tint = MaterialTheme.colorScheme.onPrimary)
             }
         }
     ) { paddingValues ->
@@ -143,13 +143,13 @@ fun EmptyState(modifier: Modifier = Modifier) {
         Text(
             "暂无任务",
             style = MaterialTheme.typography.titleLarge,
-            color = Color.Gray
+            color = MaterialTheme.colorScheme.onSurfaceVariant
         )
         Spacer(modifier = Modifier.height(8.dp))
         Text(
             "点击右下角添加第一个任务",
             style = MaterialTheme.typography.bodyMedium,
-            color = Color.Gray
+            color = MaterialTheme.colorScheme.onSurfaceVariant
         )
     }
 }

android-app/app/src/main/java/com/tunpok/calendarwidget/ui/display/DisplayScreen.kt

@@ -36,8 +36,8 @@ fun DisplayScreen(
                     )
                 },
                 colors = TopAppBarDefaults.topAppBarColors(
-                    containerColor = PrimaryColor,
-                    titleContentColor = Color.White
+                    containerColor = MaterialTheme.colorScheme.primary,
+                    titleContentColor = MaterialTheme.colorScheme.onPrimary
                 )
             )
         }
@@ -56,7 +56,7 @@ fun DisplayScreen(
                     "暂无任务",
                     modifier = Modifier.align(Alignment.Center),
                     style = MaterialTheme.typography.titleLarge,
-                    color = Color.Gray
+                    color = MaterialTheme.colorScheme.onSurfaceVariant
                 )
             } else {
                 LazyVerticalGrid(

android-app/app/src/main/java/com/tunpok/calendarwidget/ui/theme/Color.kt

@@ -10,14 +10,35 @@ val Purple40 = Color(0xFF6650a4)
 val PurpleGrey40 = Color(0xFF625b71)
 val Pink40 = Color(0xFF7D5260)
 
-// Custom colors for Calendar Widget
+// Custom colors for Calendar Widget - Light Mode
 val PrimaryColor = Color(0xFF667EEA)
 val SecondaryColor = Color(0xFF764BA2)
 val BackgroundColor = Color(0xFFF5F5F9)
 val SurfaceColor = Color(0xFFF2F2F2)
 val ErrorColor = Color(0xFFFF4757)
 
-// Task status colors
+// Task status colors - Light Mode
 val TaskGreen = Color(0xFF00B894)
 val TaskYellow = Color(0xFFFDCB6E)
 val TaskRed = Color(0xFFFF4757)
+
+// Dark Mode Colors
+val DarkPrimaryColor = Color(0xFF5E72E4)
+val DarkSecondaryColor = Color(0xFF8B5CF6)
+val DarkBackgroundColor = Color(0xFF1A1A1A)
+val DarkSurfaceColor = Color(0xFF2A2A2A)
+val DarkErrorColor = Color(0xFFFF6B6B)
+
+// Task status colors - Dark Mode
+val DarkTaskGreen = Color(0xFF4ADE80)
+val DarkTaskYellow = Color(0xFFFACC15)
+val DarkTaskRed = Color(0xFFFF6B6B)
+
+// Text colors for dark mode
+val DarkTextPrimary = Color(0xFFFFFFFF)
+val DarkTextSecondary = Color(0xFFD1D5DB)
+val DarkTextDisabled = Color(0xFF6B7280)
+
+// Card colors for dark mode
+val DarkCardBackground = Color(0xFF374151)
+val DarkDivider = Color(0xFF4B5563)

android-app/app/src/main/res/layout/widget_item_layout.xml

@@ -14,7 +14,7 @@
         android:layout_height="wrap_content"
         android:text="Task Name"
         android:textSize="14sp"
-        android:textColor="@color/black"
+        android:textColor="?android:attr/textColorPrimary"
         android:textAlignment="center"
         android:ellipsize="end"
         android:maxLines="1" />

android-app/app/src/main/res/layout/widget_layout_large.xml

@@ -19,7 +19,7 @@
             android:text="@string/app_name"
             android:textSize="16sp"
             android:textStyle="bold"
-            android:textColor="@color/black" />
+            android:textColor="?android:attr/textColorPrimary" />
         
         <ImageButton
             android:id="@+id/widget_refresh_button"

android-app/app/src/main/res/layout/widget_layout_medium.xml

@@ -19,7 +19,7 @@
             android:text="@string/app_name"
             android:textSize="14sp"
             android:textStyle="bold"
-            android:textColor="@color/black" />
+            android:textColor="?android:attr/textColorPrimary" />
         
         <ImageButton
             android:id="@+id/widget_refresh_button"

android-app/app/src/main/res/layout/widget_layout_small.xml

@@ -19,7 +19,7 @@
             android:text="@string/app_name"
             android:textSize="12sp"
             android:textStyle="bold"
-            android:textColor="@color/black" />
+            android:textColor="?android:attr/textColorPrimary" />
         
         <ImageButton
             android:id="@+id/widget_refresh_button"

android-app/app/src/main/res/values-night/colors.xml

@@ -0,0 +1,24 @@
+<?xml version="1.0" encoding="utf-8"?>
+<resources>
+    <!-- Custom colors for dark mode -->
+    <color name="primary">#FF5E72E4</color>
+    <color name="secondary">#FF8B5CF6</color>
+    <color name="background">#FF1A1A1A</color>
+    <color name="surface">#FF2A2A2A</color>
+    <color name="error">#FFFF6B6B</color>
+
+    <!-- Task status colors (adjusted for dark mode) -->
+    <color name="task_green">#FF4ADE80</color>
+    <color name="task_yellow">#FFFACC15</color>
+    <color name="task_red">#FFFF6B6B</color>
+    <color name="task_gray">#FF9CA3AF</color>
+
+    <!-- Text colors for dark mode -->
+    <color name="text_primary">#FFFFFFFF</color>
+    <color name="text_secondary">#FFD1D5DB</color>
+    <color name="text_disabled">#FF6B7280</color>
+
+    <!-- Card and surface colors -->
+    <color name="card_background">#FF374151</color>
+    <color name="divider">#FF4B5563</color>
+</resources>

android-app/app/src/main/res/values-night/themes.xml

@@ -0,0 +1,25 @@
+<?xml version="1.0" encoding="utf-8"?>
+<resources xmlns:tools="http://schemas.android.com/tools">
+    <!-- Dark theme for Calendar Widget -->
+    <style name="Theme.CalendarWidget" parent="Theme.MaterialComponents.DayNight.DarkActionBar">
+        <!-- Primary brand color. -->
+        <item name="colorPrimary">@color/primary</item>
+        <item name="colorPrimaryVariant">@color/secondary</item>
+        <item name="colorOnPrimary">@color/black</item>
+        <!-- Secondary brand color. -->
+        <item name="colorSecondary">@color/primary</item>
+        <item name="colorSecondaryVariant">@color/secondary</item>
+        <item name="colorOnSecondary">@color/black</item>
+        <!-- Background and surface colors. -->
+        <item name="android:colorBackground">@color/background</item>
+        <item name="colorSurface">@color/surface</item>
+        <item name="colorOnBackground">@color/text_primary</item>
+        <item name="colorOnSurface">@color/text_primary</item>
+        <!-- Status bar color. -->
+        <item name="android:statusBarColor">?attr/colorPrimaryVariant</item>
+        <!-- Text colors -->
+        <item name="android:textColorPrimary">@color/text_primary</item>
+        <item name="android:textColorSecondary">@color/text_secondary</item>
+        <item name="android:textColorHint">@color/text_disabled</item>
+    </style>
+</resources>