add a system theme option that follows the os light or dark setting, and make it the default for new installs

### src/main/java/com/sparrowwallet/sparrow/AboutController.java
@@ -1,5 +1,6 @@
 package com.sparrowwallet.sparrow;
 
+import com.sparrowwallet.sparrow.control.DialogImage;
 import javafx.event.ActionEvent;
 import javafx.fxml.FXML;
 import javafx.scene.control.Label;
@@ -11,10 +12,26 @@ public class AboutController {
     @FXML
     private Label title;
 
+    @FXML
+    private DialogImage dialogImage;
+
     public void initializeView() {
         title.setText(SparrowWallet.APP_NAME + " " + SparrowWallet.APP_VERSION + SparrowWallet.APP_VERSION_SUFFIX);
     }
 
+    public void refreshTheme() {
+        String darkCss = AppServices.class.getResource("darktheme.css").toExternalForm();
+        if(AppServices.isDarkTheme()) {
+            if(!stage.getScene().getStylesheets().contains(darkCss)) {
+                stage.getScene().getStylesheets().add(darkCss);
+            }
+        } else {
+            stage.getScene().getStylesheets().remove(darkCss);
+        }
+
+        dialogImage.refresh();
+    }
+
     public void setStage(Stage stage) {
         this.stage = stage;
     }

### src/main/java/com/sparrowwallet/sparrow/AppController.java
@@ -388,8 +388,8 @@ void initializeView() {
 
         Theme configTheme = Config.get().getTheme();
         if(configTheme == null) {
-            configTheme = Theme.LIGHT;
-            Config.get().setTheme(Theme.LIGHT);
+            configTheme = Theme.SYSTEM;
+            Config.get().setTheme(Theme.SYSTEM);
         }
         final Theme selectedTheme = configTheme;
         Optional<Toggle> selectedThemeToggle = theme.getToggles().stream().filter(toggle -> selectedTheme.equals(toggle.getUserData())).findFirst();
@@ -578,6 +578,8 @@ private Stage getAboutStage() {
             controller.initializeView();
             setStageIcon(stage);
             stage.setOnShowing(event -> {
+                //The macOS application menu reuses a single About stage, so the theme may have changed since it was created
+                controller.refreshTheme();
                 AppServices.moveToActiveWindowScreen(stage, 600, 460);
             });
 
@@ -1374,6 +1376,7 @@ private void addImportedWallet(Wallet wallet) {
         File walletFile = Storage.getExistingWallet(wallet.getName());
         if(walletFile != null) {
             Alert alert = new Alert(Alert.AlertType.CONFIRMATION);
+            alert.initOwner(rootStack.getScene().getWindow());
             AppServices.setStageIcon(alert.getDialogPane().getScene().getWindow());
             alert.setTitle("Existing wallet found");
             alert.setHeaderText("Replace existing wallet?");
@@ -2643,7 +2646,7 @@ public void setTheme(ActionEvent event) {
             Config.get().setTheme(selectedTheme);
         }
 
-        EventManager.get().post(new ThemeChangedEvent(selectedTheme));
+        EventManager.get().post(new ThemeChangedEvent(AppServices.getActiveTheme()));
     }
 
     private void serverToggleStartAnimation() {
@@ -2747,13 +2750,24 @@ private void setTorIcon() {
 
     @Subscribe
     public void themeChanged(ThemeChangedEvent event) {
+        //Owned dialogs follow the main window stylesheets, but these non-modal dialogs have no owner
+        List<Scene> scenes = new ArrayList<>(List.of(tabs.getScene()));
+        if(sendToManyDialog != null) {
+            scenes.add(sendToManyDialog.getDialogPane().getScene());
+        }
+        if(searchWalletDialog != null) {
+            scenes.add(searchWalletDialog.getDialogPane().getScene());
+        }
+
         String darkCss = getClass().getResource("darktheme.css").toExternalForm();
-        if(event.getTheme() == Theme.DARK) {
-            if(!tabs.getScene().getStylesheets().contains(darkCss)) {
-                tabs.getScene().getStylesheets().add(darkCss);
+        for(Scene scene : scenes) {
+            if(event.getTheme() == Theme.DARK) {
+                if(!scene.getStylesheets().contains(darkCss)) {
+                    scene.getStylesheets().add(darkCss);
+                }
+            } else {
+                scene.getStylesheets().remove(darkCss);
             }
-        } else {
-            tabs.getScene().getStylesheets().remove(darkCss);
         }
 
         for(Tab tab : tabs.getTabs()) {

### src/main/java/com/sparrowwallet/sparrow/AppServices.java
@@ -30,6 +30,7 @@
 import io.reactivex.rxjavafx.schedulers.JavaFxScheduler;
 import io.reactivex.subjects.PublishSubject;
 import javafx.application.Application;
+import javafx.application.ColorScheme;
 import javafx.application.Platform;
 import javafx.beans.property.BooleanProperty;
 import javafx.beans.property.SimpleBooleanProperty;
@@ -132,6 +133,8 @@ public class AppServices {
 
     private static volatile ChainTip announcedTip;
 
+    private static volatile boolean systemDarkTheme;
+
     private static final Map<Integer, BlockSummary> blockSummaries = new ConcurrentHashMap<>();
 
     private static Map<Integer, Double> targetBlockFeeRates;
@@ -956,13 +959,42 @@ public static Optional<ButtonType> showAlertDialog(String title, String content,
         return getInteractionServices().showAlert(title, content, alertType, graphic, buttons);
     }
 
+    public static void monitorSystemTheme() {
+        try {
+            Platform.Preferences preferences = Platform.getPreferences();
+            systemDarkTheme = preferences.getColorScheme() == ColorScheme.DARK;
+            preferences.colorSchemeProperty().addListener((observable, oldValue, colorScheme) -> {
+                systemDarkTheme = colorScheme == ColorScheme.DARK;
+                if(Config.get().getTheme() == null || Config.get().getTheme() == Theme.SYSTEM) {
+                    EventManager.get().post(new ThemeChangedEvent(getActiveTheme()));
+                }
+            });
+        } catch(Exception e) {
+            log.warn("Could not read the system color scheme", e);
+        }
+    }
+
+    public static Theme getActiveTheme() {
+        Theme theme = Config.get().getTheme();
+        if(theme == null || theme == Theme.SYSTEM) {
+            return systemDarkTheme ? Theme.DARK : Theme.LIGHT;
+        }
+
+        return theme;
+    }
+
+    public static boolean isDarkTheme() {
+        return getActiveTheme() == Theme.DARK;
+    }
+
     public static void setStageIcon(Window window) {
         Stage stage = (Stage)window;
         stage.getIcons().add(getWindowIcon());
 
         if(stage.getScene() != null) {
-            if(Config.get().getTheme() == Theme.DARK) {
-                stage.getScene().getStylesheets().add(AppServices.class.getResource("darktheme.css").toExternalForm());
+            String darkCss = AppServices.class.getResource("darktheme.css").toExternalForm();
+            if(isDarkTheme() && !stage.getScene().getStylesheets().contains(darkCss)) {
+                stage.getScene().getStylesheets().add(darkCss);
             }
             if(Config.get().isChunkAddresses()) {
                 stage.getScene().getRoot().getStyleClass().add("chunk-addresses");

### src/main/java/com/sparrowwallet/sparrow/SparrowDesktop.java
@@ -46,6 +46,7 @@ public void start(Stage stage) throws Exception {
         URL.setURLStreamHandlerFactory(protocol -> WalletIcon.PROTOCOL.equals(protocol) ? new WalletIcon.WalletIconStreamHandler() : null);
 
         AppServices.initialize(this);
+        AppServices.monitorSystemTheme();
 
         boolean createNewWallet = false;
         Mode mode = Config.get().getMode();

### src/main/java/com/sparrowwallet/sparrow/Theme.java
@@ -1,5 +1,5 @@
 package com.sparrowwallet.sparrow;
 
 public enum Theme {
-    LIGHT, DARK
+    LIGHT, DARK, SYSTEM
 }

### src/main/java/com/sparrowwallet/sparrow/control/DevicePane.java
@@ -993,6 +993,7 @@ private void discoverWallet() {
         List<Wallet> wallets = new ArrayList<>();
 
         RangeInputDialog rangeInputDialog = new RangeInputDialog(StandardAccount.ACCOUNT_0.getAccountNumber(), StandardAccount.ACCOUNT_30.getAccountNumber(), StandardAccount.ACCOUNT_10.getAccountNumber());
+        rangeInputDialog.initOwner(this.getScene().getWindow());
         rangeInputDialog.setTitle("Choose number of accounts");
         rangeInputDialog.setHeaderText("Enter the number of additional accounts to scan for existing funds.\n\nThis may take a few minutes depending on how many accounts are selected.");
         Optional<Integer> optRange = rangeInputDialog.showAndWait();

### src/main/java/com/sparrowwallet/sparrow/control/DialogImage.java
@@ -1,8 +1,6 @@
 package com.sparrowwallet.sparrow.control;
 
 import com.sparrowwallet.sparrow.AppServices;
-import com.sparrowwallet.sparrow.Theme;
-import com.sparrowwallet.sparrow.io.Config;
 import javafx.beans.NamedArg;
 import javafx.beans.property.ObjectProperty;
 import javafx.beans.property.SimpleObjectProperty;
@@ -43,7 +41,7 @@ public void refresh() {
 
     protected void refresh(Type type) {
         SVGImage svgImage;
-        if(Config.get().getTheme() == Theme.DARK) {
+        if(AppServices.isDarkTheme()) {
             svgImage = loadSVGImage("/image/dialog/" + type.name().toLowerCase(Locale.ROOT) + "-invert.svg");
         } else {
             svgImage = loadSVGImage("/image/dialog/" + type.name().toLowerCase(Locale.ROOT) + ".svg");

### src/main/java/com/sparrowwallet/sparrow/control/MempoolSizeFeeRatesChart.java
@@ -2,9 +2,7 @@
 
 import com.sparrowwallet.drongo.OsType;
 import com.sparrowwallet.sparrow.AppServices;
-import com.sparrowwallet.sparrow.Theme;
 import com.sparrowwallet.sparrow.glyphfont.FontAwesome5;
-import com.sparrowwallet.sparrow.io.Config;
 import com.sparrowwallet.sparrow.net.MempoolRateSize;
 import javafx.application.Platform;
 import javafx.beans.NamedArg;
@@ -63,7 +61,7 @@ public void handle(MouseEvent event) {
                 }
 
                 scenePane.getStylesheets().add(AppServices.class.getResource("general.css").toExternalForm());
-                if(Config.get().getTheme() == Theme.DARK) {
+                if(AppServices.isDarkTheme()) {
                     scenePane.getStylesheets().add(AppServices.class.getResource("darktheme.css").toExternalForm());
                 }
                 scenePane.getStylesheets().add(AppServices.class.getResource("wallet/wallet.css").toExternalForm());

### src/main/java/com/sparrowwallet/sparrow/control/QREncoding.java
@@ -1,9 +1,7 @@
 package com.sparrowwallet.sparrow.control;
 
 import com.sparrowwallet.sparrow.AppServices;
-import com.sparrowwallet.sparrow.Theme;
 import com.sparrowwallet.sparrow.glyphfont.FontAwesome5;
-import com.sparrowwallet.sparrow.io.Config;
 import javafx.geometry.Insets;
 import javafx.scene.Node;
 import org.controlsfx.glyphfont.Glyph;
@@ -31,7 +29,7 @@ public String getName() {
 
     public Node getSVGImage() {
         try {
-            URL url = AppServices.class.getResource("/image/qrencoding/" + getName().toLowerCase(Locale.ROOT) + "-icon" + (Config.get().getTheme() == Theme.DARK ? "-invert" : "") + ".svg");
+            URL url = AppServices.class.getResource("/image/qrencoding/" + getName().toLowerCase(Locale.ROOT) + "-icon" + (AppServices.isDarkTheme() ? "-invert" : "") + ".svg");
             if(url != null) {
                 return SVGLoader.load(url);
             } else {

### src/main/java/com/sparrowwallet/sparrow/control/TransactionDiagram.java
@@ -98,7 +98,7 @@ public void handle(MouseEvent event) {
                 }
 
                 scenePane.getStylesheets().add(AppServices.class.getResource("general.css").toExternalForm());
-                if(Config.get().getTheme() == Theme.DARK) {
+                if(AppServices.isDarkTheme()) {
                     scenePane.getStylesheets().add(AppServices.class.getResource("darktheme.css").toExternalForm());
                 }
                 scenePane.getStylesheets().add(AppServices.class.getResource("wallet/wallet.css").toExternalForm());
@@ -998,7 +998,7 @@ private void saveAsImage() {
             transactionDiagram.setFinal(true);
             transactionDiagram.setExpanded(isExpanded());
             transactionDiagram.setBackground(new Background(new BackgroundFill(Color.TRANSPARENT, null, null)));
-            transactionDiagram.setStyle("-fx-text-background-color: " + (Config.get().getTheme() == Theme.DARK ? "#ffffff" : "#000000"));
+            transactionDiagram.setStyle("-fx-text-background-color: " + (AppServices.isDarkTheme() ? "#ffffff" : "#000000"));
             updateDerivedDiagram(transactionDiagram);
             Scene scene = new Scene(transactionDiagram);
             scene.setFill(Color.TRANSPARENT);

### src/main/java/com/sparrowwallet/sparrow/control/WalletIcon.java
@@ -2,17 +2,14 @@
 
 import com.sparrowwallet.drongo.wallet.*;
 import com.sparrowwallet.sparrow.AppServices;
-import com.sparrowwallet.sparrow.Theme;
 import com.sparrowwallet.sparrow.glyphfont.FontAwesome5;
-import com.sparrowwallet.sparrow.io.Config;
 import com.sparrowwallet.sparrow.io.ImageUtils;
 import com.sparrowwallet.sparrow.io.Storage;
 import javafx.application.Platform;
 import javafx.beans.property.ObjectProperty;
 import javafx.beans.property.SimpleObjectProperty;
 import javafx.geometry.Pos;
 import javafx.scene.image.Image;
-import javafx.scene.image.ImageView;
 import javafx.scene.layout.StackPane;
 import javafx.scene.paint.ImagePattern;
 import javafx.scene.shape.Circle;
@@ -73,7 +70,7 @@ public void refresh() {
                 WalletModel walletModel = keystore.getWalletModel();
 
                 SVGImage svgImage;
-                if(Config.get().getTheme() == Theme.DARK) {
+                if(AppServices.isDarkTheme()) {
                     svgImage = loadSVGImage("/image/walletmodel/" + walletModel.getType() + "-icon-invert.svg");
                 } else {
                     svgImage = loadSVGImage("/image/walletmodel/" + walletModel.getType() + "-icon.svg");

### src/main/java/com/sparrowwallet/sparrow/control/WalletModelImage.java
@@ -2,12 +2,9 @@
 
 import com.sparrowwallet.drongo.wallet.WalletModel;
 import com.sparrowwallet.sparrow.AppServices;
-import com.sparrowwallet.sparrow.Theme;
-import com.sparrowwallet.sparrow.io.Config;
 import javafx.beans.NamedArg;
 import javafx.beans.property.ObjectProperty;
 import javafx.beans.property.SimpleObjectProperty;
-import javafx.scene.image.Image;
 import javafx.scene.layout.StackPane;
 import org.girod.javafx.svgimage.SVGImage;
 import org.girod.javafx.svgimage.SVGLoader;
@@ -51,7 +48,7 @@ public void refresh() {
 
     protected void refresh(WalletModel walletModel) {
         SVGImage svgImage;
-        if(Config.get().getTheme() == Theme.DARK) {
+        if(AppServices.isDarkTheme()) {
             svgImage = loadSVGImage("/image/walletmodel/" + walletModel.getType() + "-invert.svg");
         } else {
             svgImage = loadSVGImage("/image/walletmodel/" + walletModel.getType() + ".svg");

### src/main/java/com/sparrowwallet/sparrow/wallet/PaymentController.java
@@ -903,7 +903,7 @@ public static Glyph getPayNymGlyph() {
     public static Node getBitcoinCharacter() {
         try {
             URL url;
-            if(Config.get().getTheme() == Theme.DARK) {
+            if(AppServices.isDarkTheme()) {
                 url = AppServices.class.getResource("/image/bitcoin-character-invert.svg");
             } else {
                 url = AppServices.class.getResource("/image/bitcoin-character.svg");

### src/main/resources/com/sparrowwallet/sparrow/about.fxml
@@ -14,7 +14,7 @@
                 <Label fx:id="title" text="Sparrow" styleClass="title-label" />
             </HBox>
             <Region HBox.hgrow="ALWAYS"/>
-            <DialogImage type="SPARROW" AnchorPane.rightAnchor="0"/>
+            <DialogImage fx:id="dialogImage" type="SPARROW" AnchorPane.rightAnchor="0"/>
         </HBox>
         <VBox spacing="10" styleClass="content-area">
             <Label text="Sparrow is a Bitcoin wallet with the goal of providing greater transparency and usability on the path to full financial self-sovereignty. It attempts to provide all of the detail about your wallet setup, transactions and UTXOs so that you can transact with a full understanding of your money." wrapText="true" />

### src/main/resources/com/sparrowwallet/sparrow/app.fxml
@@ -107,6 +107,11 @@
                         </Menu>
                         <Menu mnemonicParsing="false" text="Theme">
                             <items>
+                                <RadioMenuItem mnemonicParsing="false" text="System" toggleGroup="$theme" onAction="#setTheme">
+                                    <userData>
+                                        <Theme fx:constant="SYSTEM" />
+                                    </userData>
+                                </RadioMenuItem>
                                 <RadioMenuItem mnemonicParsing="false" text="Light" toggleGroup="$theme" onAction="#setTheme">
                                     <userData>
                                         <Theme fx:constant="LIGHT" />