[SDDM Theme] Fix background in qmlscene by providing proper config dummy data

Background is Plasma blue again and contents readable for testing

Differential Revision: https://phabricator.kde.org/D3464

sddm-theme/dummydata/config.qml

@@ -1,5 +1,7 @@
 import QtQuick 2.0
 QtObject
 {
+    property string type: "color"
+    property color color: "#1d99f3" // Plasma blue
     property string background: ""
 }