SwitchLayoutEditor

In Switch Layout Editor open the .nxtheme file you want to patch

You will not see the .nxtheme file so be sure to click the dropdown box and change the settings to All files (*.*)

select the allapps.nxtheme you wish to edit

once opened click on Extract all files

the image.jpg is the background image & layout.json is the original theme json/modification file, that contains all the modification details