Set proper background color for open project dialogs.

part of CURA-9402

plugins/3MFReader/WorkspaceDialog.qml

@@ -17,6 +17,7 @@ UM.Dialog
     minimumWidth: UM.Theme.getSize("popup_dialog").width
     minimumHeight: UM.Theme.getSize("popup_dialog").height
     width: minimumWidth
+    backgroundColor: UM.Theme.getColor("main_background")
     margin: UM.Theme.getSize("default_margin").width
     property int comboboxHeight: UM.Theme.getSize("default_margin").height
 

resources/qml/Dialogs/AskOpenAsProjectOrModelsDialog.qml

@@ -17,6 +17,7 @@ UM.Dialog
     title: catalog.i18nc("@title:window", "Open project file")
     width: UM.Theme.getSize("small_popup_dialog").width
     height: UM.Theme.getSize("small_popup_dialog").height
+    backgroundColor: UM.Theme.getColor("main_background")
 
     maximumHeight: height
     maximumWidth: width