fix: use system theme to prevent color mismatch

The problem stems from using something like QtQuick.Controls.iOS in
certain views, which will follow the operating system level control.
Thus, if our app theme is dark and the human's ios system theme is
light, the text on a iOS.Menu will be white from our palette on top of a
light mode background.

Theme.qml

@@ -22,13 +22,5 @@ Item {
 
     property int radius: 5
 
-    property bool darkTheme: false
-
-    Settings {
-        property alias darkTheme: item.darkTheme
-    }
-
-    function toggleTheme() {
-        darkTheme = !darkTheme;
-    }
+    property bool darkTheme: Application.styleHints.colorScheme === Qt.Dark
 }

gallery/Main.qml

@@ -25,12 +25,6 @@ AppWindow {
             Row {
                 spacing: 5
 
-                AppPrimaryButton {
-                    text: "Toggle theme"
-                    onClicked: {
-                        Theme.toggleTheme();
-                    }
-                }
                 AppSecondaryButton {
                     text: "Secondary"
                 }