PowerShell returning Null value for current applied theme of publishing site collection SP2013

Hi All,

I want to get the current applied theme of a site collection using Powershell. I used below code, but it is returning null values

          $spWeb = Get-SPWeb -Identity http://sp2013/sites/composedlooks
          [Microsoft.SharePoint.Utilities.ThmxTheme]::GetThemeUrlForWeb($spWeb.RootWeb)

I also tried to use "[Microsoft.SharePoint.Utilities.ThmxTheme]::GetManagedThemes" to get all themes in theme gallery. This is also returning null.

I'm not sure where I did wrong. Site collection is ofpublishing type. I applied theme to it from 'Change the Look' option in site settings

I tested by applying default available theme and also by custom theme. In both cases, current theme is returned as null.

Can anyone please let me know where it wrong