OK I finished the second one, so now both the Curia and HRE windows use the correct DG artwork.
See >this post< on previous page for the files.
Note that mod\Dei Gratia\gfx\interface\hre_bg.pdn isn't referenced, hence should be safe to exclude from your archive. (I have no idea what it is...?)
:edit: OK the .PDN file is an old version of the vanilla HRE background in Paint.net image format - definitely scrap.