The background was easy: replace ID 4300. But the Scene needed a workaround, in definition 947
code:
<*PlusSkin Conditions(RemoveSceneLogon = false) *><Element Id=atom(idSceneBackground) LayoutPos=top Layout=FillLayout()>
<WallpaperView Id=atom(idScene) Thumbnail=false Active=0 Enabled=false Height=rcint("msgr",654) AddBehavior=DUI::ShowHide()/>
</Element>
<Element LayoutPos=top id=atom(idSceneEdge) Background=rcbkd("msgr",4301) Height=rcint("msgr",4111) AddBehavior=DUI::ShowHide()/></*PlusSkin*>
<*PlusSkin Conditions(RemoveSceneLogon = true) *><Element Id=atom(idSceneBackground) LayoutPos=none Layout=FillLayout()>
<WallpaperView Id=atom(idScene) Thumbnail=false Active=0 Enabled=false Height=rcint("msgr",654) AddBehavior=DUI::ShowHide()/>
</Element>
<Element LayoutPos=none id=atom(idSceneEdge) Background=rcbkd("msgr",4301) Height=rcint("msgr",4111) AddBehavior=DUI::ShowHide()/></*PlusSkin*>
(note that scene can be removed with a BoolVal and backgrounds can be changed with Multivals)
And that white field in the midle, isn't messenger's default, but ID 4302