feat: add dark/light mode toggle with system/light/dark options

app/src/main/java/com/swoosh/microblog/MainActivity.kt

@@ -4,13 +4,20 @@ import android.os.Bundle
 import androidx.activity.ComponentActivity
 import androidx.activity.compose.setContent
 import androidx.activity.enableEdgeToEdge
+import androidx.activity.viewModels
+import androidx.lifecycle.compose.collectAsStateWithLifecycle
+import androidx.compose.runtime.getValue
 import androidx.navigation.compose.rememberNavController
 import com.swoosh.microblog.data.CredentialsManager
 import com.swoosh.microblog.ui.navigation.Routes
 import com.swoosh.microblog.ui.navigation.SwooshNavGraph
 import com.swoosh.microblog.ui.theme.SwooshTheme
+import com.swoosh.microblog.ui.theme.ThemeViewModel
 
 class MainActivity : ComponentActivity() {
+
+    private val themeViewModel: ThemeViewModel by viewModels()
+
     override fun onCreate(savedInstanceState: Bundle?) {
         super.onCreate(savedInstanceState)
         enableEdgeToEdge()
@@ -19,11 +26,14 @@ class MainActivity : ComponentActivity() {
         val startDestination = if (credentials.isConfigured) Routes.FEED else Routes.SETUP
 
         setContent {
-            SwooshTheme {
+            val themeMode by themeViewModel.themeMode.collectAsStateWithLifecycle()
+
+            SwooshTheme(themeMode = themeMode) {
                 val navController = rememberNavController()
                 SwooshNavGraph(
                     navController = navController,
-                    startDestination = startDestination
+                    startDestination = startDestination,
+                    themeViewModel = themeViewModel
                 )
             }
         }

app/src/main/java/com/swoosh/microblog/ui/feed/FeedScreen.kt

@@ -8,6 +8,9 @@ import androidx.compose.foundation.lazy.rememberLazyListState
 import androidx.compose.material.ExperimentalMaterialApi
 import androidx.compose.material.icons.Icons
 import androidx.compose.material.icons.filled.Add
+import androidx.compose.material.icons.filled.BrightnessAuto
+import androidx.compose.material.icons.filled.DarkMode
+import androidx.compose.material.icons.filled.LightMode
 import androidx.compose.material.icons.filled.Refresh
 import androidx.compose.material.icons.filled.Settings
 import androidx.compose.material.icons.filled.WifiOff
@@ -28,6 +31,8 @@ import androidx.lifecycle.viewmodel.compose.viewModel
 import coil.compose.AsyncImage
 import com.swoosh.microblog.data.model.FeedPost
 import com.swoosh.microblog.data.model.QueueStatus
+import com.swoosh.microblog.ui.theme.ThemeMode
+import com.swoosh.microblog.ui.theme.ThemeViewModel
 
 @OptIn(ExperimentalMaterial3Api::class, ExperimentalMaterialApi::class)
 @Composable
@@ -35,9 +40,11 @@ fun FeedScreen(
     onSettingsClick: () -> Unit,
     onPostClick: (FeedPost) -> Unit,
     onCompose: () -> Unit,
-    viewModel: FeedViewModel = viewModel()
+    viewModel: FeedViewModel = viewModel(),
+    themeViewModel: ThemeViewModel? = null
 ) {
     val state by viewModel.uiState.collectAsStateWithLifecycle()
+    val themeMode = themeViewModel?.themeMode?.collectAsStateWithLifecycle()
     val listState = rememberLazyListState()
 
     // Pull-to-refresh
@@ -65,6 +72,17 @@ fun FeedScreen(
             TopAppBar(
                 title = { Text("Swoosh") },
                 actions = {
+                    if (themeViewModel != null) {
+                        val currentMode = themeMode?.value ?: ThemeMode.SYSTEM
+                        val (icon, description) = when (currentMode) {
+                            ThemeMode.SYSTEM -> Icons.Default.BrightnessAuto to "Theme: System"
+                            ThemeMode.LIGHT -> Icons.Default.LightMode to "Theme: Light"
+                            ThemeMode.DARK -> Icons.Default.DarkMode to "Theme: Dark"
+                        }
+                        IconButton(onClick = { themeViewModel.cycleThemeMode() }) {
+                            Icon(icon, contentDescription = description)
+                        }
+                    }
                     IconButton(onClick = { viewModel.refresh() }) {
                         Icon(Icons.Default.Refresh, contentDescription = "Refresh")
                     }

app/src/main/java/com/swoosh/microblog/ui/navigation/NavGraph.kt

@@ -13,6 +13,7 @@ import com.swoosh.microblog.ui.feed.FeedScreen
 import com.swoosh.microblog.ui.feed.FeedViewModel
 import com.swoosh.microblog.ui.settings.SettingsScreen
 import com.swoosh.microblog.ui.setup.SetupScreen
+import com.swoosh.microblog.ui.theme.ThemeViewModel
 
 object Routes {
     const val SETUP = "setup"
@@ -25,7 +26,8 @@ object Routes {
 @Composable
 fun SwooshNavGraph(
     navController: NavHostController,
-    startDestination: String
+    startDestination: String,
+    themeViewModel: ThemeViewModel
 ) {
     // Shared state for passing posts between screens
     var selectedPost by remember { mutableStateOf<FeedPost?>(null) }
@@ -47,6 +49,7 @@ fun SwooshNavGraph(
         composable(Routes.FEED) {
             FeedScreen(
                 viewModel = feedViewModel,
+                themeViewModel = themeViewModel,
                 onSettingsClick = { navController.navigate(Routes.SETTINGS) },
                 onPostClick = { post ->
                     selectedPost = post
@@ -91,6 +94,7 @@ fun SwooshNavGraph(
 
         composable(Routes.SETTINGS) {
             SettingsScreen(
+                themeViewModel = themeViewModel,
                 onBack = { navController.popBackStack() },
                 onLogout = {
                     navController.navigate(Routes.SETUP) {

app/src/main/java/com/swoosh/microblog/ui/settings/SettingsScreen.kt

@@ -1,24 +1,34 @@
 package com.swoosh.microblog.ui.settings
 
 import androidx.compose.foundation.layout.*
+import androidx.compose.foundation.rememberScrollState
 import androidx.compose.foundation.text.KeyboardOptions
+import androidx.compose.foundation.verticalScroll
 import androidx.compose.material.icons.Icons
 import androidx.compose.material.icons.automirrored.filled.ArrowBack
+import androidx.compose.material.icons.filled.BrightnessAuto
+import androidx.compose.material.icons.filled.DarkMode
+import androidx.compose.material.icons.filled.LightMode
 import androidx.compose.material3.*
 import androidx.compose.runtime.*
+import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.platform.LocalContext
 import androidx.compose.ui.text.input.KeyboardType
 import androidx.compose.ui.text.input.PasswordVisualTransformation
 import androidx.compose.ui.unit.dp
+import androidx.lifecycle.compose.collectAsStateWithLifecycle
 import com.swoosh.microblog.data.CredentialsManager
 import com.swoosh.microblog.data.api.ApiClient
+import com.swoosh.microblog.ui.theme.ThemeMode
+import com.swoosh.microblog.ui.theme.ThemeViewModel
 
 @OptIn(ExperimentalMaterial3Api::class)
 @Composable
 fun SettingsScreen(
     onBack: () -> Unit,
-    onLogout: () -> Unit
+    onLogout: () -> Unit,
+    themeViewModel: ThemeViewModel? = null
 ) {
     val context = LocalContext.current
     val credentials = remember { CredentialsManager(context) }
@@ -26,6 +36,8 @@ fun SettingsScreen(
     var apiKey by remember { mutableStateOf(credentials.adminApiKey ?: "") }
     var saved by remember { mutableStateOf(false) }
 
+    val currentThemeMode = themeViewModel?.themeMode?.collectAsStateWithLifecycle()
+
     Scaffold(
         topBar = {
             TopAppBar(
@@ -43,7 +55,24 @@ fun SettingsScreen(
                 .fillMaxSize()
                 .padding(padding)
                 .padding(24.dp)
+                .verticalScroll(rememberScrollState())
         ) {
+            // --- Appearance section ---
+            if (themeViewModel != null) {
+                Text("Appearance", style = MaterialTheme.typography.titleMedium)
+                Spacer(modifier = Modifier.height(12.dp))
+
+                ThemeModeSelector(
+                    currentMode = currentThemeMode?.value ?: ThemeMode.SYSTEM,
+                    onModeSelected = { themeViewModel.setThemeMode(it) }
+                )
+
+                Spacer(modifier = Modifier.height(24.dp))
+                HorizontalDivider()
+                Spacer(modifier = Modifier.height(24.dp))
+            }
+
+            // --- Ghost Instance section ---
             Text("Ghost Instance", style = MaterialTheme.typography.titleMedium)
             Spacer(modifier = Modifier.height(16.dp))
 
@@ -91,7 +120,7 @@ fun SettingsScreen(
             }
 
             Spacer(modifier = Modifier.height(32.dp))
-            Divider()
+            HorizontalDivider()
             Spacer(modifier = Modifier.height(16.dp))
 
             OutlinedButton(
@@ -110,3 +139,37 @@ fun SettingsScreen(
         }
     }
 }
+
+@Composable
+fun ThemeModeSelector(
+    currentMode: ThemeMode,
+    onModeSelected: (ThemeMode) -> Unit
+) {
+    val modes = listOf(
+        Triple(ThemeMode.SYSTEM, "System", Icons.Default.BrightnessAuto),
+        Triple(ThemeMode.LIGHT, "Light", Icons.Default.LightMode),
+        Triple(ThemeMode.DARK, "Dark", Icons.Default.DarkMode)
+    )
+
+    Row(
+        modifier = Modifier.fillMaxWidth(),
+        horizontalArrangement = Arrangement.spacedBy(8.dp)
+    ) {
+        modes.forEach { (mode, label, icon) ->
+            val selected = currentMode == mode
+            FilterChip(
+                selected = selected,
+                onClick = { onModeSelected(mode) },
+                label = { Text(label) },
+                leadingIcon = {
+                    Icon(
+                        imageVector = icon,
+                        contentDescription = null,
+                        modifier = Modifier.size(18.dp)
+                    )
+                },
+                modifier = Modifier.weight(1f)
+            )
+        }
+    }
+}

app/src/main/java/com/swoosh/microblog/ui/theme/Theme.kt

@@ -8,10 +8,16 @@ import androidx.compose.ui.platform.LocalContext
 
 @Composable
 fun SwooshTheme(
-    darkTheme: Boolean = isSystemInDarkTheme(),
+    themeMode: ThemeMode = ThemeMode.SYSTEM,
     dynamicColor: Boolean = true,
     content: @Composable () -> Unit
 ) {
+    val darkTheme = when (themeMode) {
+        ThemeMode.SYSTEM -> isSystemInDarkTheme()
+        ThemeMode.LIGHT -> false
+        ThemeMode.DARK -> true
+    }
+
     val colorScheme = when {
         dynamicColor && Build.VERSION.SDK_INT >= Build.VERSION_CODES.S -> {
             val context = LocalContext.current

app/src/main/java/com/swoosh/microblog/ui/theme/ThemeMode.kt

@@ -0,0 +1,19 @@
+package com.swoosh.microblog.ui.theme
+
+/**
+ * Represents the user's theme preference.
+ */
+enum class ThemeMode {
+    SYSTEM,
+    LIGHT,
+    DARK;
+
+    /**
+     * Returns the next mode in the cycle: SYSTEM -> LIGHT -> DARK -> SYSTEM.
+     */
+    fun next(): ThemeMode = when (this) {
+        SYSTEM -> LIGHT
+        LIGHT -> DARK
+        DARK -> SYSTEM
+    }
+}

app/src/main/java/com/swoosh/microblog/ui/theme/ThemePreferences.kt

@@ -0,0 +1,34 @@
+package com.swoosh.microblog.ui.theme
+
+import android.content.Context
+import android.content.SharedPreferences
+
+/**
+ * Persists the user's theme preference to SharedPreferences.
+ * Uses regular SharedPreferences (not encrypted) since theme preference is not sensitive.
+ */
+class ThemePreferences(context: Context) {
+
+    private val prefs: SharedPreferences = context.getSharedPreferences(
+        PREFS_NAME,
+        Context.MODE_PRIVATE
+    )
+
+    var themeMode: ThemeMode
+        get() {
+            val name = prefs.getString(KEY_THEME_MODE, ThemeMode.SYSTEM.name)
+            return try {
+                ThemeMode.valueOf(name ?: ThemeMode.SYSTEM.name)
+            } catch (e: IllegalArgumentException) {
+                ThemeMode.SYSTEM
+            }
+        }
+        set(value) {
+            prefs.edit().putString(KEY_THEME_MODE, value.name).apply()
+        }
+
+    companion object {
+        const val PREFS_NAME = "swoosh_theme_prefs"
+        const val KEY_THEME_MODE = "theme_mode"
+    }
+}

app/src/main/java/com/swoosh/microblog/ui/theme/ThemeViewModel.kt

@@ -0,0 +1,31 @@
+package com.swoosh.microblog.ui.theme
+
+import android.app.Application
+import androidx.lifecycle.AndroidViewModel
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.flow.asStateFlow
+
+/**
+ * App-level ViewModel that manages theme mode state.
+ * Should be scoped to the Activity so all screens share the same instance.
+ */
+class ThemeViewModel(application: Application) : AndroidViewModel(application) {
+
+    private val themePreferences = ThemePreferences(application)
+
+    private val _themeMode = MutableStateFlow(themePreferences.themeMode)
+    val themeMode: StateFlow<ThemeMode> = _themeMode.asStateFlow()
+
+    fun setThemeMode(mode: ThemeMode) {
+        themePreferences.themeMode = mode
+        _themeMode.value = mode
+    }
+
+    /**
+     * Cycles through modes: SYSTEM -> LIGHT -> DARK -> SYSTEM.
+     */
+    fun cycleThemeMode() {
+        setThemeMode(_themeMode.value.next())
+    }
+}

app/src/test/java/com/swoosh/microblog/ui/theme/ThemeModeTest.kt

@@ -0,0 +1,70 @@
+package com.swoosh.microblog.ui.theme
+
+import org.junit.Assert.*
+import org.junit.Test
+
+class ThemeModeTest {
+
+    @Test
+    fun `ThemeMode has exactly 3 values`() {
+        assertEquals(3, ThemeMode.values().size)
+    }
+
+    @Test
+    fun `ThemeMode values are SYSTEM, LIGHT, DARK`() {
+        val values = ThemeMode.values().toList()
+        assertEquals(ThemeMode.SYSTEM, values[0])
+        assertEquals(ThemeMode.LIGHT, values[1])
+        assertEquals(ThemeMode.DARK, values[2])
+    }
+
+    @Test
+    fun `ThemeMode valueOf works for all values`() {
+        assertEquals(ThemeMode.SYSTEM, ThemeMode.valueOf("SYSTEM"))
+        assertEquals(ThemeMode.LIGHT, ThemeMode.valueOf("LIGHT"))
+        assertEquals(ThemeMode.DARK, ThemeMode.valueOf("DARK"))
+    }
+
+    @Test(expected = IllegalArgumentException::class)
+    fun `ThemeMode valueOf throws for invalid value`() {
+        ThemeMode.valueOf("INVALID")
+    }
+
+    // --- Cycling logic ---
+
+    @Test
+    fun `next from SYSTEM returns LIGHT`() {
+        assertEquals(ThemeMode.LIGHT, ThemeMode.SYSTEM.next())
+    }
+
+    @Test
+    fun `next from LIGHT returns DARK`() {
+        assertEquals(ThemeMode.DARK, ThemeMode.LIGHT.next())
+    }
+
+    @Test
+    fun `next from DARK returns SYSTEM`() {
+        assertEquals(ThemeMode.SYSTEM, ThemeMode.DARK.next())
+    }
+
+    @Test
+    fun `cycling through all modes returns to start`() {
+        val start = ThemeMode.SYSTEM
+        val afterFirst = start.next()
+        val afterSecond = afterFirst.next()
+        val afterThird = afterSecond.next()
+        assertEquals(start, afterThird)
+    }
+
+    @Test
+    fun `full cycle from LIGHT returns to LIGHT`() {
+        val start = ThemeMode.LIGHT
+        assertEquals(start, start.next().next().next())
+    }
+
+    @Test
+    fun `full cycle from DARK returns to DARK`() {
+        val start = ThemeMode.DARK
+        assertEquals(start, start.next().next().next())
+    }
+}

app/src/test/java/com/swoosh/microblog/ui/theme/ThemePreferencesTest.kt

@@ -0,0 +1,92 @@
+package com.swoosh.microblog.ui.theme
+
+import android.app.Application
+import android.content.Context
+import androidx.test.core.app.ApplicationProvider
+import org.junit.Assert.*
+import org.junit.Before
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.annotation.Config
+
+@RunWith(RobolectricTestRunner::class)
+@Config(application = Application::class)
+class ThemePreferencesTest {
+
+    private lateinit var context: Context
+    private lateinit var themePreferences: ThemePreferences
+
+    @Before
+    fun setUp() {
+        context = ApplicationProvider.getApplicationContext()
+        // Clear prefs before each test
+        context.getSharedPreferences(ThemePreferences.PREFS_NAME, Context.MODE_PRIVATE)
+            .edit().clear().commit()
+        themePreferences = ThemePreferences(context)
+    }
+
+    @Test
+    fun `default theme mode is SYSTEM`() {
+        assertEquals(ThemeMode.SYSTEM, themePreferences.themeMode)
+    }
+
+    @Test
+    fun `set and get LIGHT mode`() {
+        themePreferences.themeMode = ThemeMode.LIGHT
+        assertEquals(ThemeMode.LIGHT, themePreferences.themeMode)
+    }
+
+    @Test
+    fun `set and get DARK mode`() {
+        themePreferences.themeMode = ThemeMode.DARK
+        assertEquals(ThemeMode.DARK, themePreferences.themeMode)
+    }
+
+    @Test
+    fun `set and get SYSTEM mode`() {
+        themePreferences.themeMode = ThemeMode.DARK
+        themePreferences.themeMode = ThemeMode.SYSTEM
+        assertEquals(ThemeMode.SYSTEM, themePreferences.themeMode)
+    }
+
+    @Test
+    fun `persists across instances`() {
+        themePreferences.themeMode = ThemeMode.DARK
+        val newInstance = ThemePreferences(context)
+        assertEquals(ThemeMode.DARK, newInstance.themeMode)
+    }
+
+    @Test
+    fun `overwriting mode replaces previous value`() {
+        themePreferences.themeMode = ThemeMode.LIGHT
+        themePreferences.themeMode = ThemeMode.DARK
+        assertEquals(ThemeMode.DARK, themePreferences.themeMode)
+    }
+
+    @Test
+    fun `invalid stored value falls back to SYSTEM`() {
+        // Write an invalid value directly to SharedPreferences
+        context.getSharedPreferences(ThemePreferences.PREFS_NAME, Context.MODE_PRIVATE)
+            .edit().putString(ThemePreferences.KEY_THEME_MODE, "INVALID_VALUE").commit()
+        val prefs = ThemePreferences(context)
+        assertEquals(ThemeMode.SYSTEM, prefs.themeMode)
+    }
+
+    @Test
+    fun `null stored value falls back to SYSTEM`() {
+        // Remove the key from SharedPreferences
+        context.getSharedPreferences(ThemePreferences.PREFS_NAME, Context.MODE_PRIVATE)
+            .edit().remove(ThemePreferences.KEY_THEME_MODE).commit()
+        val prefs = ThemePreferences(context)
+        assertEquals(ThemeMode.SYSTEM, prefs.themeMode)
+    }
+
+    @Test
+    fun `all modes can be round-tripped`() {
+        for (mode in ThemeMode.values()) {
+            themePreferences.themeMode = mode
+            assertEquals(mode, themePreferences.themeMode)
+        }
+    }
+}