Sidebar UI updated.

src/main/java/org/to/telegramfinalproject/UI/ThemeManager.java

@@ -0,0 +1,58 @@
+package org.to.telegramfinalproject.UI;
+
+import javafx.beans.property.BooleanProperty;
+import javafx.beans.property.SimpleBooleanProperty;
+import javafx.scene.Scene;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class ThemeManager {
+    private static final ThemeManager instance = new ThemeManager();
+
+    private final BooleanProperty darkMode = new SimpleBooleanProperty(false);
+    private final List<Scene> registeredScenes = new ArrayList<>();
+
+    private ThemeManager() {
+        // Whenever darkMode changes, update all registered scenes
+        darkMode.addListener((obs, oldVal, newVal) -> applyThemeToAll());
+    }
+
+    public static ThemeManager getInstance() {
+        return instance;
+    }
+
+    public BooleanProperty darkModeProperty() {
+        return darkMode;
+    }
+
+    public boolean isDarkMode() {
+        return darkMode.get();
+    }
+
+    public void setDarkMode(boolean dark) {
+        darkMode.set(dark);
+    }
+
+    public void registerScene(Scene scene) {
+        if (!registeredScenes.contains(scene)) {
+            registeredScenes.add(scene);
+            applyTheme(scene); // Apply current theme immediately
+        }
+    }
+
+    private void applyThemeToAll() {
+        for (Scene scene : registeredScenes) {
+            applyTheme(scene);
+        }
+    }
+
+    private void applyTheme(Scene scene) {
+        scene.getStylesheets().clear();
+        if (isDarkMode()) {
+            scene.getStylesheets().add(getClass().getResource("/org/to/telegramfinalproject/CSS/dark_theme.css").toExternalForm());
+        } else {
+            scene.getStylesheets().add(getClass().getResource("/org/to/telegramfinalproject/CSS/light_theme.css").toExternalForm());
+        }
+    }
+}