I suppose the migration has preserved the names of the themes in the database you can check in table sf_page_templates. To change the currently selected themes please run this API code in the Page_Load method of a webform.
var tManager = PageManager.GetManager();
var templates = tManager.GetTemplates().Where(t => t.Title ==
// put the name of the template
var draft = tManager.EditTemplate(t.Id);
//name of the theme that should be used
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the Telerik team
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