Here is the code snippet that will help.

$webAppUrl = "<yourWebAppUrl>"
$webapp = Get-SPWebApplication $webAppUrl  | Get-SPSite -Limit All | Get-SPSite -limit all | ForEach-Object {Enable-SPFeature -Identity 'Your feature guid'  -Url $_.Url}

Recently, I had to create this PowerShell command to activate branding features to all the existing site collections.