To apply a theme to SharePoint site in SharePoint 2010 environment ,we can use the following Power Shell commands.
$Site = “http://mysite/sites”
$Web = Get-SPWeb -Site $Site
$theme = [Microsoft.SharePoint.Utilities.ThmxTheme]::Open($site,"_catalogs/theme/mytheme.thmx")
$theme.ApplyTo($web,$false)
$web.Update()
$web.Dispose()
$site.Dispose()
*mytheme is a customized theme that we are applying.