fanis · fanis · Aug 9, 2026 · Aug 9, 2026
diff --git a/android_app/app/src/main/java/dev/fanis/expensenotification/BaseActivity.java b/android_app/app/src/main/java/dev/fanis/expensenotification/BaseActivity.java
@@ -2,8 +2,10 @@
 
 import android.annotation.SuppressLint;
 import android.app.Activity;
+import android.content.Context;
 import android.content.Intent;
 import android.content.res.ColorStateList;
+import android.content.res.Configuration;
 import android.graphics.Color;
 import android.graphics.Insets;
 import android.graphics.Typeface;
@@ -26,20 +28,69 @@
 
 /** Shared UI scaffolding and battery-optimization helpers for the app's screens. */
 abstract class BaseActivity extends Activity {
-    protected static final int COLOR_TEAL = 0xff13696a;
-    protected static final int COLOR_BG = 0xfff6f8f9;
-    protected static final int COLOR_CARD = 0xffffffff;
-    protected static final int COLOR_BORDER = 0xffd6dde2;
-    protected static final int COLOR_TEXT = 0xff0d1924;
-    protected static final int COLOR_MUTED = 0xff485460;
-    protected static final int COLOR_DANGER = 0xff9f2f44;
+    // Palette fields keep their constant-era names; they are resolved per activity
+    // in attachBaseContext once the effective light/dark mode is known.
+    protected int COLOR_TEAL = 0xff13696a;
+    protected int COLOR_TEAL_TEXT = 0xff13696a;
+    protected int COLOR_BG = 0xfff6f8f9;
+    protected int COLOR_CARD = 0xffffffff;
+    protected int COLOR_BORDER = 0xffd6dde2;
+    protected int COLOR_TEXT = 0xff0d1924;
+    protected int COLOR_MUTED = 0xff485460;
+    protected int COLOR_HINT = 0xff788692;
+    protected int COLOR_DANGER = 0xff9f2f44;
+
+    private String appliedThemeMode;
+    private boolean darkMode;
+
+    @Override
+    protected void attachBaseContext(Context newBase) {
+        super.attachBaseContext(newBase);
+        appliedThemeMode = ThemePreferences.mode(newBase);
+        int forcedNight = ThemePreferences.forcedNightBits(appliedThemeMode);
+        if (forcedNight != 0) {
+            // Overriding uiMode makes this activity's resources (values-night theme,
+            // dialogs) and the isDarkMode() check below follow the forced choice.
+            Configuration override = new Configuration();
+            int baseUiMode = newBase.getResources().getConfiguration().uiMode;
+            override.uiMode = forcedNight | (baseUiMode & ~Configuration.UI_MODE_NIGHT_MASK);
+            applyOverrideConfiguration(override);
+        }
+        darkMode = (getResources().getConfiguration().uiMode & Configuration.UI_MODE_NIGHT_MASK)
+                == Configuration.UI_MODE_NIGHT_YES;
+        if (darkMode) {
+            COLOR_TEAL_TEXT = 0xff7ecfd0;
+            COLOR_BG = 0xff10171c;
+            COLOR_CARD = 0xff1a232a;
+            COLOR_BORDER = 0xff32404a;
+            COLOR_TEXT = 0xffe8edf1;
+            COLOR_MUTED = 0xffa4b2bc;
+            COLOR_HINT = 0xff6f7f8b;
+            COLOR_DANGER = 0xffb43a52;
+        }
+    }
+
+    @Override
+    protected void onResume() {
+        super.onResume();
+        // A stacked activity resumes with a stale palette after the theme changed
+        // in Settings; rebuild it with the new choice applied.
+        if (appliedThemeMode != null && !appliedThemeMode.equals(ThemePreferences.mode(this))) {
+            recreate();
+        }
+    }
+
+    protected boolean isDarkMode() {
+        return darkMode;
+    }
 
     /** Wraps a vertical content column in a scroller with status/navigation-bar insets applied. */
     protected ScrollView scrollRoot(LinearLayout root) {
         Window window = getWindow();
         window.setStatusBarColor(COLOR_TEAL);
-        window.setNavigationBarColor(Color.WHITE);
-        window.getDecorView().setSystemUiVisibility(View.SYSTEM_UI_FLAG_LIGHT_NAVIGATION_BAR);
+        window.setNavigationBarColor(darkMode ? COLOR_BG : Color.WHITE);
+        window.getDecorView().setSystemUiVisibility(
+                darkMode ? 0 : View.SYSTEM_UI_FLAG_LIGHT_NAVIGATION_BAR);
         ScrollView scroll = new ScrollView(this);
         scroll.setFillViewport(true);
         scroll.setBackgroundColor(COLOR_BG);
@@ -198,7 +249,7 @@ protected Button button(String label) {
 
     protected Button secondaryButton(String label) {
         Button button = button(label);
-        button.setTextColor(COLOR_TEAL);
+        button.setTextColor(COLOR_TEAL_TEXT);
         button.setBackground(rounded(COLOR_CARD, COLOR_BORDER, 5));
         return button;
     }
@@ -252,7 +303,7 @@ protected void styleCard(LinearLayout card) {
 
     protected void styleInput(EditText input) {
         input.setTextColor(COLOR_TEXT);
-        input.setHintTextColor(0xff788692);
+        input.setHintTextColor(COLOR_HINT);
         input.setTextSize(15);
         input.setBackgroundTintList(ColorStateList.valueOf(COLOR_BORDER));
     }

diff --git a/android_app/app/src/main/java/dev/fanis/expensenotification/SettingsActivity.java b/android_app/app/src/main/java/dev/fanis/expensenotification/SettingsActivity.java
@@ -19,6 +19,7 @@ public class SettingsActivity extends BaseActivity {
 
     private CandidateDb db;
     private Button currencyButton;
+    private Button themeButton;
     private Button payeeAliasesButton;
     private Button batteryButton;
 
@@ -72,6 +73,10 @@ private View buildUi() {
         currencyButton.setOnClickListener(v -> showCurrencyDialog());
         root.addView(currencyButton);
 
+        themeButton = button(themeLabel());
+        themeButton.setOnClickListener(v -> showThemeDialog());
+        root.addView(themeButton);
+
         payeeAliasesButton = button(payeeAliasesLabel());
         payeeAliasesButton.setOnClickListener(v -> startActivity(new Intent(this, PayeeAliasesActivity.class)));
         root.addView(payeeAliasesButton);
@@ -166,6 +171,44 @@ private void showCurrencyDialog() {
                 .show();
     }
 
+    private String themeLabel() {
+        return "Theme: " + ThemePreferences.label(ThemePreferences.mode(this));
+    }
+
+    private void showThemeDialog() {
+        String[] modes = new String[]{
+                ThemePreferences.MODE_AUTO,
+                ThemePreferences.MODE_LIGHT,
+                ThemePreferences.MODE_DARK,
+        };
+        CharSequence[] labels = new CharSequence[modes.length];
+        int currentIndex = 0;
+        String current = ThemePreferences.mode(this);
+        for (int i = 0; i < modes.length; i++) {
+            labels[i] = ThemePreferences.label(modes[i]);
+            if (modes[i].equals(current)) {
+                currentIndex = i;
+            }
+        }
+        new AlertDialog.Builder(this)
+                .setTitle("Theme")
+                .setSingleChoiceItems(labels, currentIndex, (dialog, which) -> {
+                    boolean changed = !modes[which].equals(current);
+                    ThemePreferences.setMode(this, modes[which]);
+                    if (themeButton != null) {
+                        themeButton.setText(themeLabel());
+                    }
+                    dialog.dismiss();
+                    if (changed) {
+                        // Rebuild this screen with the new palette; stacked
+                        // activities catch up in BaseActivity.onResume.
+                        recreate();
+                    }
+                })
+                .setNegativeButton("Cancel", null)
+                .show();
+    }
+
     private String payeeAliasesLabel() {
         int blocked = PayeeAliases.blacklistCount(this);
         String label = "Payee aliases (" + PayeeAliases.count(this) + ")";

diff --git a/android_app/app/src/main/java/dev/fanis/expensenotification/ThemePreferences.java b/android_app/app/src/main/java/dev/fanis/expensenotification/ThemePreferences.java
@@ -0,0 +1,55 @@
+package dev.fanis.expensenotification;
+
+import android.content.Context;
+import android.content.SharedPreferences;
+import android.content.res.Configuration;
+
+/** Stores the user's theme choice: follow the system, or force light/dark. */
+final class ThemePreferences {
+    static final String MODE_AUTO = "AUTO";
+    static final String MODE_LIGHT = "LIGHT";
+    static final String MODE_DARK = "DARK";
+
+    private static final String PREFS = "theme_preferences";
+    private static final String KEY_MODE = "mode";
+
+    private ThemePreferences() {
+    }
+
+    static String mode(Context context) {
+        String value = prefs(context).getString(KEY_MODE, MODE_AUTO);
+        if (MODE_LIGHT.equals(value) || MODE_DARK.equals(value)) {
+            return value;
+        }
+        return MODE_AUTO;
+    }
+
+    static void setMode(Context context, String mode) {
+        prefs(context).edit().putString(KEY_MODE, mode).apply();
+    }
+
+    static String label(String mode) {
+        if (MODE_LIGHT.equals(mode)) {
+            return "Light";
+        }
+        if (MODE_DARK.equals(mode)) {
+            return "Dark";
+        }
+        return "Auto (follow system)";
+    }
+
+    /** The UI_MODE_NIGHT_* bits the given mode forces, or 0 when following the system. */
+    static int forcedNightBits(String mode) {
+        if (MODE_LIGHT.equals(mode)) {
+            return Configuration.UI_MODE_NIGHT_NO;
+        }
+        if (MODE_DARK.equals(mode)) {
+            return Configuration.UI_MODE_NIGHT_YES;
+        }
+        return 0;
+    }
+
+    private static SharedPreferences prefs(Context context) {
+        return context.getSharedPreferences(PREFS, Context.MODE_PRIVATE);
+    }
+}
diff --git a/android_app/app/src/main/res/values-night/styles.xml b/android_app/app/src/main/res/values-night/styles.xml
@@ -0,0 +1,7 @@
+<resources>
+    <style name="AppTheme" parent="android:style/Theme.Material.NoActionBar">
+        <item name="android:fontFamily">sans</item>
+        <item name="android:windowLightStatusBar">false</item>
+        <item name="android:colorAccent">#7fb3ef</item>
+    </style>
+</resources>
diff --git a/..._app/app/src/test/java/dev/fanis/expensenotification/ThemePreferencesRobolectricTest.java b/..._app/app/src/test/java/dev/fanis/expensenotification/ThemePreferencesRobolectricTest.java
@@ -0,0 +1,59 @@
+package dev.fanis.expensenotification;
+
+import static org.junit.Assert.assertEquals;
+
+import android.content.Context;
+import android.content.res.Configuration;
+
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.robolectric.RobolectricTestRunner;
+import org.robolectric.RuntimeEnvironment;
+
+@RunWith(RobolectricTestRunner.class)
+public class ThemePreferencesRobolectricTest {
+
+    private Context context;
+
+    @Before
+    public void setUp() {
+        context = RuntimeEnvironment.getApplication();
+    }
+
+    @Test
+    public void defaultsToAuto() {
+        assertEquals(ThemePreferences.MODE_AUTO, ThemePreferences.mode(context));
+    }
+
+    @Test
+    public void persistsSelectedMode() {
+        ThemePreferences.setMode(context, ThemePreferences.MODE_DARK);
+        assertEquals(ThemePreferences.MODE_DARK, ThemePreferences.mode(context));
+
+        ThemePreferences.setMode(context, ThemePreferences.MODE_LIGHT);
+        assertEquals(ThemePreferences.MODE_LIGHT, ThemePreferences.mode(context));
+    }
+
+    @Test
+    public void unknownStoredValueFallsBackToAuto() {
+        ThemePreferences.setMode(context, "PURPLE");
+        assertEquals(ThemePreferences.MODE_AUTO, ThemePreferences.mode(context));
+    }
+
+    @Test
+    public void forcedNightBitsMatchModes() {
+        assertEquals(0, ThemePreferences.forcedNightBits(ThemePreferences.MODE_AUTO));
+        assertEquals(Configuration.UI_MODE_NIGHT_NO,
+                ThemePreferences.forcedNightBits(ThemePreferences.MODE_LIGHT));
+        assertEquals(Configuration.UI_MODE_NIGHT_YES,
+                ThemePreferences.forcedNightBits(ThemePreferences.MODE_DARK));
+    }
+
+    @Test
+    public void labelsAreHumanReadable() {
+        assertEquals("Auto (follow system)", ThemePreferences.label(ThemePreferences.MODE_AUTO));
+        assertEquals("Light", ThemePreferences.label(ThemePreferences.MODE_LIGHT));
+        assertEquals("Dark", ThemePreferences.label(ThemePreferences.MODE_DARK));
+    }
+}