Set-WindowsTheme.ps1

function Set-WindowsTheme {
  param([ValidateSet('Light', 'Dark')] $Value)

  foreach ($path in 'HKLM:\SOFTWARE\Microsoft\Windows\CurrentVersion\Themes\Personalize',
           'HKCU:\SOFTWARE\Microsoft\Windows\CurrentVersion\Themes\Personalize')
  {
    $personalize = if (Test-Path $path) {
      Get-Item $path
    } else {
      New-Item $path
    }

    $propertyValue = if ($Value -eq 'Light') { 1 } else { 0 }
    if ($personalize.PSObject.Properties['AppsUseLightTheme'] -eq $null) {
      Set-ItemProperty -Path $path -Name AppsUseLightTheme -Value $propertyValue
    } else {
      New-ItemProperty -Path $path -Name AppsUseLightTheme -Value $propertyValue -PropertyType DWORD |
	      Out-Null
    }
  }
}