Reset-SiteToDefaultOfficeTheme.ps1

Add-PSSnapin Microsoft.SharePoint.PowerShell

function Remove-ComposedLook
{
    [cmdletbinding()]
    param
    (
        [Parameter(Mandatory=$true)][string]$Name,    
        [Parameter(Mandatory=$true)][Microsoft.SharePoint.SPWeb]$Web    
    )

    $designGallery = $web.GetCatalog([Microsoft.SharePoint.SPListTemplateType]::DesignCatalog)

    if( $designGallery )
    {
        $composedLooks = $designGallery.GetItems( 
            (New-Object Microsoft.SharePoint.SPQuery -Property @{
                ViewXml = "<View><Query><Where><Eq><FieldRef Name='Name' /><Value Type='Text'>$Name</Value></Eq></Where></Query></View>"
                ViewFields = "<FieldRef Name='Name'/>"
                ViewFieldsOnly = $true
            }))

        for($x = 0; $x -lt $composedLooks.Count; $x++ )
        {
            $composedLooks[$x].Delete()
        }
    }
}

$allsites = Get-SPSite -Limit All 

foreach( $site in $allsites )
{
    # remove the composed look called "Current" from each web in the site collection
    $site| Get-SPweb -Limit All | % { Remove-ComposedLook -Web $_ -Name "Current" }

    # reset the site back to the default "office" theme on each web in the site collection
    $site| Get-SPweb -Limit All | % { [Microsoft.SharePoint.Utilities.ThmxTheme]::RemoveThemeFromWeb($_, $true <# delete theme files #> ) }
}