Respect QGCPalette and ui design

src/QmlControls/QGCComboBox.qml

 import QtQuick 2.2
 import QtQuick.Controls 1.2
 import QtQuick.Controls.Styles 1.2
+import QtQuick.Controls.Private 1.0
 
 import QGroundControl.Palette 1.0
 
 ComboBox {
-    property var __palette: QGCPalette { colorGroupEnabled: enabled }
+    property var __qgcPal: QGCPalette { colorGroupEnabled: enabled }
 
+    property bool __showHighlight: pressed | hovered
+
+    style: ComboBoxStyle {
+        textColor: __showHighlight ?
+                    control.__qgcPal.buttonHighlightText :
+                    control.__qgcPal.buttonText
+
+        background: Item {
+            implicitWidth: Math.round(TextSingleton.implicitHeight * 4.5)
+            implicitHeight: Math.max(25, Math.round(TextSingleton.implicitHeight * 1.2))
+
+            Rectangle {
+                anchors.fill: parent
+                color: __showHighlight ?
+                    control.__qgcPal.buttonHighlight :
+                    control.__qgcPal.button
+            }
+
+            Image {
+                id: imageItem
+                visible: control.menu !== null
+                source: "arrow-down.png"
+                anchors.verticalCenter: parent.verticalCenter
+                anchors.right: parent.right
+                anchors.rightMargin: dropDownButtonWidth / 2
+                opacity: control.enabled ? 0.6 : 0.3
+            }
+        }
+    }
 }