Here is another post about how to customize your SharePoint 2010 site. The last one was about how to change the CSS of your SharePoint Site by PowerShell.
This script shows you how you can change the Theme of your site using PowerShell.
$mainurl = “http://vlad.test.loc”
$themeName = “Azur”
$site = new-object Microsoft.SharePoint.SPSite($mainurl)
$web = $site.OpenWeb()
$theme = [Microsoft.SharePoint.Utilities.ThmxTheme]::Open($site,”_catalogs/theme/” + $themeName + “.thmx”)
Enjoy and Say Thanks if it helped :)!