Follow system theme on Android 10+.

app/src/main/java/org/thoughtcrime/securesms/preferences/AppearancePreferenceFragment.java

@@ -7,6 +7,7 @@ import androidx.preference.ListPreference;
 
 import org.thoughtcrime.securesms.ApplicationPreferencesActivity;
 import org.thoughtcrime.securesms.R;
+import org.thoughtcrime.securesms.util.DynamicTheme;
 import org.thoughtcrime.securesms.util.TextSecurePreferences;
 
 import java.util.Arrays;

app/src/main/java/org/thoughtcrime/securesms/util/DynamicDarkActionBarTheme.java

@@ -1,18 +1,16 @@
 package org.thoughtcrime.securesms.util;
 
-import android.app.Activity;
+import androidx.annotation.StyleRes;
 
 import org.thoughtcrime.securesms.R;
 
 public class DynamicDarkActionBarTheme extends DynamicTheme {
-  @Override
-  protected int getSelectedTheme(Activity activity) {
-    String theme = TextSecurePreferences.getTheme(activity);
-
-    if (theme.equals("dark")) {
-      return R.style.TextSecure_DarkTheme_Conversation;
-    }
 
+  protected @StyleRes int getLightThemeStyle() {
     return R.style.TextSecure_LightTheme_Conversation;
   }
+
+  protected @StyleRes int getDarkThemeStyle() {
+    return R.style.TextSecure_DarkTheme_Conversation;
+  }
 }

app/src/main/java/org/thoughtcrime/securesms/util/DynamicDarkToolbarTheme.java

@@ -1,18 +1,16 @@
 package org.thoughtcrime.securesms.util;
 
-import android.app.Activity;
+import androidx.annotation.StyleRes;
 
 import org.thoughtcrime.securesms.R;
 
 public class DynamicDarkToolbarTheme extends DynamicTheme {
-  @Override
-  protected int getSelectedTheme(Activity activity) {
-    String theme = TextSecurePreferences.getTheme(activity);
-
-    if (theme.equals("dark")) {
-      return R.style.TextSecure_DarkNoActionBar_DarkToolbar;
-    }
 
+  protected @StyleRes int getLightThemeStyle() {
     return R.style.TextSecure_LightNoActionBar_DarkToolbar;
   }
+
+  protected @StyleRes int getDarkThemeStyle() {
+    return R.style.TextSecure_DarkNoActionBar_DarkToolbar;
+  }
 }

app/src/main/java/org/thoughtcrime/securesms/util/DynamicIntroTheme.java

@@ -1,16 +1,16 @@
 package org.thoughtcrime.securesms.util;
 
-import android.app.Activity;
+import androidx.annotation.StyleRes;
 
 import org.thoughtcrime.securesms.R;
 
 public class DynamicIntroTheme extends DynamicTheme {
-  @Override
-  protected int getSelectedTheme(Activity activity) {
-    String theme = TextSecurePreferences.getTheme(activity);
-
-    if (theme.equals("dark")) return R.style.TextSecure_DarkIntroTheme;
 
+  protected @StyleRes int getLightThemeStyle() {
     return R.style.TextSecure_LightIntroTheme;
   }
+
+  protected @StyleRes int getDarkThemeStyle() {
+    return R.style.TextSecure_DarkIntroTheme;
+  }
 }

app/src/main/java/org/thoughtcrime/securesms/util/DynamicNoActionBarInviteTheme.java

@@ -1,16 +1,16 @@
 package org.thoughtcrime.securesms.util;
 
-import android.app.Activity;
+import androidx.annotation.StyleRes;
 
 import org.thoughtcrime.securesms.R;
 
 public class DynamicNoActionBarInviteTheme extends DynamicTheme {
-  @Override
-  protected int getSelectedTheme(Activity activity) {
-    String theme = TextSecurePreferences.getTheme(activity);
-
-    if (theme.equals("dark")) return R.style.Signal_NoActionBar_Invite;
 
+  protected @StyleRes int getLightThemeStyle() {
     return R.style.Signal_Light_NoActionBar_Invite;
   }
+
+  protected @StyleRes int getDarkThemeStyle() {
+    return R.style.Signal_NoActionBar_Invite;
+  }
 }

app/src/main/java/org/thoughtcrime/securesms/util/DynamicNoActionBarTheme.java

@@ -1,16 +1,16 @@
 package org.thoughtcrime.securesms.util;
 
-import android.app.Activity;
+import androidx.annotation.StyleRes;
 
 import org.thoughtcrime.securesms.R;
 
 public class DynamicNoActionBarTheme extends DynamicTheme {
-  @Override
-  protected int getSelectedTheme(Activity activity) {
-    String theme = TextSecurePreferences.getTheme(activity);
-
-    if (theme.equals("dark")) return R.style.TextSecure_DarkNoActionBar;
 
+  protected @StyleRes int getLightThemeStyle() {
     return R.style.TextSecure_LightNoActionBar;
   }
+
+  protected @StyleRes int getDarkThemeStyle() {
+    return R.style.TextSecure_DarkNoActionBar;
+  }
 }

app/src/main/java/org/thoughtcrime/securesms/util/DynamicRegistrationTheme.java

@@ -1,16 +1,16 @@
 package org.thoughtcrime.securesms.util;
 
-import android.app.Activity;
+import androidx.annotation.StyleRes;
 
 import org.thoughtcrime.securesms.R;
 
 public class DynamicRegistrationTheme extends DynamicTheme {
-  @Override
-  protected int getSelectedTheme(Activity activity) {
-    String theme = TextSecurePreferences.getTheme(activity);
-
-    if (theme.equals("dark")) return R.style.TextSecure_DarkRegistrationTheme;
 
+  protected @StyleRes int getLightThemeStyle() {
     return R.style.TextSecure_LightRegistrationTheme;
   }
+
+  protected @StyleRes int getDarkThemeStyle() {
+    return R.style.TextSecure_DarkRegistrationTheme;
+  }
 }

app/src/main/java/org/thoughtcrime/securesms/util/DynamicTheme.java

@@ -3,15 +3,19 @@ package org.thoughtcrime.securesms.util;
 import android.app.Activity;
 import android.content.Intent;
 import android.content.res.Configuration;
+import android.os.Build;
 
+import androidx.annotation.NonNull;
+import androidx.annotation.StyleRes;
 import androidx.appcompat.app.AppCompatDelegate;
 
 import org.thoughtcrime.securesms.R;
 
 public class DynamicTheme {
 
-  public static final String DARK  = "dark";
+  public static final String DARK   = "dark";
-  public static final String LIGHT = "light";
+  public static final String LIGHT  = "light";
+  public static final String SYSTEM = "system";
 
   private int currentTheme;
 
@@ -30,16 +34,38 @@ public class DynamicTheme {
     }
   }
 
-  protected int getSelectedTheme(Activity activity) {
+  private @StyleRes int getSelectedTheme(Activity activity) {
     String theme = TextSecurePreferences.getTheme(activity);
 
-    if (theme.equals(DARK)) {
+    if (theme.equals(SYSTEM) && systemThemeAvailable()) {
-      return R.style.TextSecure_DarkTheme;
+      if (isSystemInDarkTheme(activity)) {
+        return getDarkThemeStyle();
+      } else {
+        return getLightThemeStyle();
+      }
+    } else if (theme.equals(DARK)) {
+      return getDarkThemeStyle();
+    } else {
+      return getLightThemeStyle();
     }
+  }
 
+  protected @StyleRes int getLightThemeStyle() {
     return R.style.TextSecure_LightTheme;
   }
 
+  protected @StyleRes int getDarkThemeStyle() {
+    return R.style.TextSecure_DarkTheme;
+  }
+
+  public static boolean systemThemeAvailable() {
+    return Build.VERSION.SDK_INT >= 29;
+  }
+
+  private static boolean isSystemInDarkTheme(@NonNull Activity activity) {
+    return (activity.getResources().getConfiguration().uiMode & Configuration.UI_MODE_NIGHT_MASK) == Configuration.UI_MODE_NIGHT_YES;
+  }
+
   private static final class OverridePendingTransition {
     static void invoke(Activity activity) {
       activity.overridePendingTransition(0, 0);

app/src/main/java/org/thoughtcrime/securesms/util/TextSecurePreferences.java

@@ -886,7 +886,7 @@ public class TextSecurePreferences {
   }
 
   public static String getTheme(Context context) {
-    return getStringPreference(context, THEME_PREF, "light");
+    return getStringPreference(context, THEME_PREF, DynamicTheme.systemThemeAvailable() ? DynamicTheme.SYSTEM : DynamicTheme.LIGHT);
   }
 
   public static boolean isVerifying(Context context) {

app/src/main/res/values-v29/arrays.xml

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="utf-8"?>
+<resources xmlns:tools="http://schemas.android.com/tools">
+
+  <string-array name="pref_theme_entries" tools:ignore="InconsistentArrays">
+    <item>@string/preferences__system_theme</item>
+    <item>@string/preferences__light_theme</item>
+    <item>@string/preferences__dark_theme</item>
+  </string-array>
+
+  <string-array name="pref_theme_values" translatable="false" tools:ignore="InconsistentArrays">
+    <item>system</item>
+    <item>light</item>
+    <item>dark</item>
+  </string-array>
+
+</resources>

app/src/main/res/values/arrays.xml

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="utf-8"?>
-<resources>
+<resources xmlns:tools="http://schemas.android.com/tools">
 
   <string-array name="language_entries">
 <!-- zz --><item>@string/preferences__default</item>
@@ -143,12 +143,12 @@
       <item>vi</item>
   </string-array>
 
-    <string-array name="pref_theme_entries">
+  <string-array name="pref_theme_entries" tools:ignore="InconsistentArrays">
       <item>@string/preferences__light_theme</item>
       <item>@string/preferences__dark_theme</item>
   </string-array>
 
-  <string-array name="pref_theme_values" translatable="false">
+  <string-array name="pref_theme_values" translatable="false" tools:ignore="InconsistentArrays">
       <item>light</item>
       <item>dark</item>
   </string-array>

app/src/main/res/values/strings.xml

@@ -1471,7 +1471,7 @@
     <string name="preferences__linked_devices">Linked devices</string>
     <string name="preferences__light_theme">Light</string>
     <string name="preferences__dark_theme">Dark</string>
-    <string name="preferences__system_theme">System</string>
+    <string name="preferences__system_theme">System default</string>
     <string name="preferences__appearance">Appearance</string>
     <string name="preferences__theme">Theme</string>
     <string name="preferences__default">Default</string>

app/src/main/res/xml-v29/preferences_appearance.xml

@@ -0,0 +1,18 @@
+<?xml version="1.0" encoding="utf-8"?>
+
+<PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android">
+    <org.thoughtcrime.securesms.preferences.widgets.SignalListPreference
+            android:key="pref_theme"
+            android:title="@string/preferences__theme"
+            android:entries="@array/pref_theme_entries"
+            android:entryValues="@array/pref_theme_values"
+            android:defaultValue="system">
+    </org.thoughtcrime.securesms.preferences.widgets.SignalListPreference>
+
+    <org.thoughtcrime.securesms.preferences.widgets.SignalListPreference
+            android:key="pref_language"
+            android:title="@string/preferences__language"
+            android:entries="@array/language_entries"
+            android:entryValues="@array/language_values"
+            android:defaultValue="zz"/>
+</PreferenceScreen>