Fix theme on native light mode, implement volume slider

Player.qml

@@ -5,6 +5,13 @@ pragma Singleton
 MediaPlayer {
     source: ""
 
+    audioOutput: AudioOutput {
+        id: output
+        volume: 0.4
+    }
+    
+    property alias volume: output.volume
+
     function startPlaying(url) {
         if (playing) {
             return;
@@ -20,10 +27,6 @@ MediaPlayer {
         stop();
     }
 
-    audioOutput: AudioOutput {
-        volume: 0.4
-    }
-
     onErrorOccurred: function(error, errorString) {
         stopPlaying();
     }