Is there a powershell command to determine the name of the theme is used by webs under a site collection? If yes, is there a powershell way to dissassociate the theme from a web or all the webs under a given site collection?
You can attach the object model via Powershell. There are no native cmdlets.
You can then loop thru the collection to see all the themes assigned.
You can call various methods to manage your themes in this way.
These are disposable object so you need to manage them or use the Start / Stop-SPAssigment
Vijai has posted some other examples for working with themes: http://www.go4sharepoint.com/Code/remove-sharepoint-site-theme-powershell-318.aspx
Here is a reference on how to set it automatically with code, it could probably be tweaked to reset it to Default.