This script will loop through all of the user profiles on the local machine and reset the desktop background to the default value in the registry (user hive). Win7 to Win10 USMT doesn't copy over custom wallpapers. So this is often left blank, leaving users with a black background. This is also a good example of how to edit user hive files throu…

Set-DesktopBackgroundtoDefault.ps1

<# 
*************************************************************
* Set Desktoop Background to Default for All Local Profiles *
*************************************************************

Purpose:
This script will loop through all of the user profiles on the
local machine and reset the desktop background to the default value in the registry (user hive).
Win7 to Win10 USMT doesn't copy over custom wallpapers.
So this is often left blank, leaving users with a black background.

This is also a good example of how to edit user hive files in PowerShell.
#>

#Find all the user profiles in the registry
$users = Get-ChildItem "C:\Users"

#Loop through each profile hive and set the default background
foreach($user in $users)
{
    $ntPath = ($user.FullName + "\NTUSER.DAT")
    $hiveName = ("HKU\" + $user.Name)
    $userName = $user.Name
    reg.exe load $hiveName $ntPath

    #reset the user's desktop background to default
    reg.exe add "`"HKEY_USERS\$userName\Control Panel\Desktop\"`" /v Wallpaper /t REG_SZ /f /d 'C:\WINDOWS\web\wallpaper\Windows\img0.jpg'

    #Uncomment this section if you'd like to reset the theme settings as well:
    <#
    reg.exe add "HKEY_USERS\$userName\Software\Microsoft\Windows\CurrentVersion\Themes\" /v ColorSetFromTheme /t REG_DWORD /f /d 1
    reg.exe add "HKEY_USERS\$userName\Software\Microsoft\Windows\CurrentVersion\Themes\" /v CurrentTheme /t REG_SZ /f /d "C:\WINDOWS\resources\Themes\aero.theme"
    reg.exe add "HKEY_USERS\$userName\Software\Microsoft\Windows\CurrentVersion\Themes\" /v InstallTheme /t REG_EXPAND_SZ /f /d "%SystemRoot%\resources\Themes\aero.theme"
    reg.exe add "HKEY_USERS\$userName\Software\Microsoft\Windows\CurrentVersion\Themes\" /v InstallVisualStyle /t REG_EXPAND_SZ /f /d "%ResourceDir%\themes\Aero\Aero.msstyles"
    reg.exe add "HKEY_USERS\$userName\Software\Microsoft\Windows\CurrentVersion\Themes\" /v InstallVisualStyleColor /t REG_SZ /f /d "NormalColor"
    reg.exe add "HKEY_USERS\$userName\Software\Microsoft\Windows\CurrentVersion\Themes\" /v InstallVisualStyleSize /t REG_SZ /f /d "NormalSize"
    reg.exe add "HKEY_USERS\$userName\Software\Microsoft\Windows\CurrentVersion\Themes\" /v SetupVersion /t REG_SZ /f /d "10"
    reg.exe add "HKEY_USERS\$userName\Software\Microsoft\Windows\CurrentVersion\Themes\" /v ThemeChangesDesktopIcons /t REG_DWORD /f /d 1
    reg.exe add "HKEY_USERS\$userName\Software\Microsoft\Windows\CurrentVersion\Themes\" /v ThemeChangesMousePointers /t REG_DWORD /f /d 1
    reg.exe add "HKEY_USERS\$userName\Software\Microsoft\Windows\CurrentVersion\Themes\" /v WallpaperSetFromTheme /t REG_DWORD /f /d 1
    reg.exe add "HKEY_USERS$userName\Software\Microsoft\Windows\CurrentVersion\Themes" /v HighContrast /t REG_SZ /f /d "C:\WINDOWS\resources\Themes\aero.theme"
    reg.exe add "HKEY_USERS$userName\Software\Microsoft\Windows\CurrentVersion\Themes\History\Colors" /v ColorHistory0 /t REG_DWORD /f /d 0x002987cc
    reg.exe add "HKEY_USERS$userName\Software\Microsoft\Windows\CurrentVersion\Explorer\Wallpapers" /v BackgroundType /t REG_DWORD /f /d 0x00000000
    reg.exe add "HKEY_USERS$userName\Software\Microsoft\Windows\CurrentVersion\Explorer\Accent" /v AccentPalette /t REG_BINARY /f /d a6d8ff0076b9ed00429ce3000078d700005a9e000042750000264200f7630c00
    reg.exe add "HKEY_USERS$userName\Software\Microsoft\Windows\CurrentVersion\Explorer\Accent" /v StartColorMenu /t REG_DWORD /f /d 0xff9e5a00
    reg.exe add "HKEY_USERS$userName\Software\Microsoft\Windows\CurrentVersion\Explorer\Accent" /v AccentColorMenu /t REG_DWORD /f /d 0xffd77800
    reg.exe add "HKEY_USERS$userName\Software\Microsoft\Windows\DWM" /v ColorizationColor /t REG_DWORD /f /d 0xc40078d7
    reg.exe add "HKEY_USERS$userName\Software\Microsoft\Windows\DWM" /v ColorizationAfterglow /t REG_DWORD /f /d 0xc40078d7
    reg.exe add "HKEY_USERS$userName\Software\Microsoft\Windows\DWM" /v AccentColor /t REG_DWORD /f /d 0xffd77800
    #>
    
    [gc]::Collect()
    #Unload ntuser.dat
    reg unload "$hiveName"
}

Thanks a lot - it is working great for setting Wallpaper.
Unfortunately resetting Theme does not work

With the following additions resetting the Theme to Windows Default is now working as well - not sure if every entry is really necessary:

reg.exe add "HKEY_USERS$userName\Software\Microsoft\Windows\CurrentVersion\Themes" /v HighContrast /t REG_SZ /f /d "C:\WINDOWS\resources\Themes\aero.theme"
reg.exe add "HKEY_USERS$userName\Software\Microsoft\Windows\CurrentVersion\Themes\History\Colors" /v ColorHistory0 /t REG_DWORD /f /d 0x002987cc
reg.exe add "HKEY_USERS$userName\Software\Microsoft\Windows\CurrentVersion\Explorer\Wallpapers" /v BackgroundType /t REG_DWORD /f /d 0x00000000
reg.exe add "HKEY_USERS$userName\Software\Microsoft\Windows\CurrentVersion\Explorer\Accent" /v AccentPalette /t REG_BINARY /f /d a6d8ff0076b9ed00429ce3000078d700005a9e000042750000264200f7630c00
reg.exe add "HKEY_USERS$userName\Software\Microsoft\Windows\CurrentVersion\Explorer\Accent" /v StartColorMenu /t REG_DWORD /f /d 0xff9e5a00
reg.exe add "HKEY_USERS$userName\Software\Microsoft\Windows\CurrentVersion\Explorer\Accent" /v AccentColorMenu /t REG_DWORD /f /d 0xffd77800
reg.exe add "HKEY_USERS$userName\Software\Microsoft\Windows\DWM" /v ColorizationColor /t REG_DWORD /f /d 0xc40078d7
reg.exe add "HKEY_USERS$userName\Software\Microsoft\Windows\DWM" /v ColorizationAfterglow /t REG_DWORD /f /d 0xc40078d7
reg.exe add "HKEY_USERS$userName\Software\Microsoft\Windows\DWM" /v AccentColor /t REG_DWORD /f /d 0xffd77800

That's awesome, thanks for the fix @mchwalt ! I'm sorry it took me 4 months to get back to you.