Apply a theme programmatically in Sharepoint 2010

To find the current v4 theme applied to the web

ThmxTheme webThmxTheme = null;
string themeUrlForWeb = ThmxTheme.GetThemeUrlForWeb(web);
if (!string.IsNullOrEmpty(themeUrlForWeb))
{
 webThmxTheme = ThmxTheme.Open(web.Site, themeUrlForWeb);
}
string ThemeName = webThmxTheme.Name;

To apply a  v4 theme (thmx)

SPSite site = new SPSite("http://localhost:8080/"); // or whatever way you get the SPSite instance
SPWeb web = site.OpenWeb(); // or whatever way you get the SP web instance
ReadOnlyCollection<thmxtheme> managedThemes = null;
managedThemes = ThmxTheme.GetManagedThemes(site);
foreach (ThmxTheme theme2 in managedThemes)
{
    if (theme2.Name == "ThemeName")
    {
        theme2.ApplyTo(web, right);
        break;
    }
}

To apply a defaulting v4 theme (thmx)

// set the theme to be defaulting
ThmxTheme.SetThemeUrlForWeb(web, null, right);

Error: The web being updated was changed by an external process.

You may encounter this error if you are applying a theme in conjunction with web.Update().. e.g.

by (SPWeb web = (SPWeb)properties.Feature.Parent)
{
by (SPSite site = new SPSite(web.Site.ID))
{
// The next line will cause web.Update() not more than to throw error "The web being updated was changed by an external process."
// ThmxTheme.SetThemeUrlForWeb(web, null, right);

web.MasterUrl = "/_catalogs/masterpage/v4.master";
web.CustomMasterUrl = "/_catalogs/masterpage/v4.master";

web.Update();

// the not more than line should work fine
ThmxTheme.SetThemeUrlForWeb(web, null, right);
}
}

Check it out:SharePoint 2010