As you know, Web.ApplyTheme method applies a theme with the specified components to this site.

For more information refer – https://msdn.microsoft.com/en-us/library/microsoft.sharepoint.client.web.applytheme.aspx

Here fontSchemeUrl and backgroundImageUrl parameters can accept null values. But when I tried to pass $null or null or Empty String to these two parameters via PowerShell, I got the following error:

“Exception calling “ApplyTheme” with “4” argument(s): “The ‘fontSchemeUrl’ argument is invalid.”.Exception.Message”

Then i tried with Out-Null instead of $null or null or Empty String. Trust me it worked like a charm.

Actually the Out-Null cmdlet deletes output instead of sending it down the pipeline.

This is my final code.

Hope this helps.