diff --git a/scanslicer/main.py b/scanslicer/main.py
index e3c2889..391a082 100644
--- a/scanslicer/main.py
+++ b/scanslicer/main.py
@@ -43,3 +43,52 @@ def _unwrap_qvariant(value):
                 continue
         break
     return unwrapped
+
+
+# _read_portal_dark_preference: function definition for ScanSlicer behavior.
+def _read_portal_dark_preference():
+    try:
+        from PySide6.QtDBus import QDBusConnection, QDBusInterface, QDBusMessage
+    except Exception:
+        return None
+
+    bus = QDBusConnection.sessionBus()
+    if not bus.isConnected():
+        return None
+
+    iface = QDBusInterface(
+        "org.freedesktop.portal.Desktop",
+        "/org/freedesktop/portal/desktop",
+        "org.freedesktop.portal.Settings",
+        bus,
+    )
+    if not iface.isValid():
+        return None
+
+    for method in ("ReadOne", "Read"):
+        reply = iface.call(method, "org.freedesktop.appearance", "color-scheme")
+        if reply.type() == QDBusMessage.MessageType.ErrorMessage:
+            continue
+
+        args = reply.arguments()
+        if not args:
+            continue
+
+        raw_value = _unwrap_qvariant(args[0])
+        if isinstance(raw_value, dict) and "color-scheme" in raw_value:
+            raw_value = _unwrap_qvariant(raw_value["color-scheme"])
+        if isinstance(raw_value, (list, tuple)) and raw_value:
+            raw_value = _unwrap_qvariant(raw_value[0])
+
+        try:
+            color_scheme = int(raw_value)
+        except Exception:
+            continue
+
+        # XDG portal color-scheme values:
+        # 0 = no preference, 1 = prefer dark, 2 = prefer light
+        if color_scheme == 1:
+            return True
+        if color_scheme == 2:
+            return False
+    return None