InheritMasterPage

$site = Get-SPSite -Identity "http://YourSiteCollectionUrl"

try {
  $site | Get-SPWeb -limit all | ForEach-Object {
    $_.CustomMasterUrl = $site.RootWeb.CustomMasterUrl
    $_.AllProperties["__InheritsCustomMasterUrl"] = "True"
    $_.MasterUrl = $site.RootWeb.MasterUrl
    $_.AllProperties["__InheritsMasterUrl"] = "True"
    $_.AlternateCssUrl = $site.RootWeb.AlternateCssUrl
    $_.AllProperties["__InheritsAlternateCssUrl"] = "True"
    $_.Update()
  }
}
catch [Exception]
{
    Write-Host $_.Exception.Message -ForegroundColor red
}