diff --git a/scanslicer/main.py b/scanslicer/main.py
index 391a082..db0408c 100644
--- a/scanslicer/main.py
+++ b/scanslicer/main.py
@@ -92,3 +92,40 @@ def _read_portal_dark_preference():
         if color_scheme == 2:
             return False
     return None
+
+
+# apply_dark_theme_if_requested: function definition for ScanSlicer behavior.
+def apply_dark_theme_if_requested(app):
+    theme_value = os.environ.get("GTK_THEME", "").lower()
+    dark_theme_requested = "dark" in theme_value
+    if not dark_theme_requested:
+        portal_pref = _read_portal_dark_preference()
+        if portal_pref is not None:
+            dark_theme_requested = portal_pref
+
+    if not dark_theme_requested:
+        try:
+            dark_theme_requested = app.styleHints().colorScheme() == Qt.ColorScheme.Dark
+        except Exception:
+            dark_theme_requested = False
+
+    if not dark_theme_requested:
+        return
+
+    # Some container/desktop combinations do not propagate GTK dark theme
+    # correctly to Qt widgets; enforce a dark palette when dark mode is requested.
+    app.setStyle("Fusion")
+    palette = QPalette()
+    palette.setColor(QPalette.Window, QColor(53, 53, 53))
+    palette.setColor(QPalette.WindowText, QColor(255, 255, 255))
+    palette.setColor(QPalette.Base, QColor(35, 35, 35))
+    palette.setColor(QPalette.AlternateBase, QColor(53, 53, 53))
+    palette.setColor(QPalette.ToolTipBase, QColor(35, 35, 35))
+    palette.setColor(QPalette.ToolTipText, QColor(255, 255, 255))
+    palette.setColor(QPalette.Text, QColor(255, 255, 255))
+    palette.setColor(QPalette.Button, QColor(53, 53, 53))
+    palette.setColor(QPalette.ButtonText, QColor(255, 255, 255))
+    palette.setColor(QPalette.BrightText, QColor(255, 0, 0))
+    palette.setColor(QPalette.Highlight, QColor(42, 130, 218))
+    palette.setColor(QPalette.HighlightedText, QColor(255, 255, 255))
+    app.setPalette(palette)