In order to change the back-end theme , you can simply change the css NopAdmin>Views>Shared>_AdminLayout.cshtml
<!--These need to be at the end of the page to ensure that all the controls requiring scripts have been rendered-->
@MvcHtmlString.Create(HttpUtility.HtmlDecode(Html.Telerik().StyleSheetRegistrar().DefaultGroup(group => group
.Add("2011.2.712/telerik.common.css")
.Add("2011.2.712/telerik.black.css")@*Just change the css file which can be found in root\Presentation\Nop.Web\Content\2011.2.712 Folder*@
.Add("2011.2.712/telerik.rtl.css")).ToHtmlString()))
@(MvcHtmlString.Create(HttpUtility.HtmlDecode(Html.Telerik().ScriptRegistrar()
.jQuery(false).ToHtmlString())))
Currenlty There about 10-20 themes