Find an remove unused scss variables through basic heuristic. Not particularly fast, but it seems to work okay.

Find.ps1

$ourContent = ls $PSScriptRoot\MyCode -r -fi *.scss | cat
$vars = $ourContent | % {
    if($_ -match '^\s*\$([a-zA-Z_-]+)\s*:') {
        $Matches[1]
    }
} | select -Unique

$allContent = ls $PSScriptRoot -r -fi *.scss | cat
$vars | ? {
    $search = "\`$$_[^:]*$"
    $used = $allContent | % {
        if($_ -match $search) {
            $true
        }
    }
    if(!$used) {
        write-host "`$$_"
    }
}

Remove.ps1

$disused = cat $PSScriptRoot\disused.txt

function ContainingDisused($line, $disused)
{
    foreach($d in $disused)
    {
        if($line -match "\$d") {
            return $true
        }
    }
}

ls $PSScriptRoot\NewSite\App -r -fi *.scss | % {
    $originalContent = cat $_.FullName
    $content = $originalContent | ? { !(ContainingDisused $_ $disused) }
    if(($content | Out-String) -ne ($originalContent | Out-String)) {
        [System.IO.File]::WriteAllLines($_.FullName, $content)
    }
}