# cd D:\SM\Temp\theme
Get-ChildItem -Recurse -File | ForEach-Object {
    $lower = $_.Name.ToLower()
    Rename-Item -LiteralPath $_.FullName -NewName $lower
    if ($_.Name -ne $lower) {
        Write-Host "Renaming $($_.Name) to $lower"
        Rename-Item -LiteralPath $_.FullName -NewName $lower
    }
}