RadThemeManager cannot load the theme if it is saved as package (tssp file). This is shown in the following article: Using Custom Themes
Changing Themes at Run Time with Telerik UI for for WinForms
In this video, you will learn how to give your users the ability to choose between Telerik themes and custom themes at run time. (Runtime: 08:42)
Themes can be saved as XML files using the VisualStyleBuilder tool: Saving and Loading Theme Files
To load a theme from an external file into a Theme Manager, follow these steps:
Add a RadThemeManager control to your form.
Select the RadThemeManager control, and then open its Smart Tag menu.
Select Load Theme From File.
Find your theme in the Open File dialog box, and then click Open.
It is up to you to choose a theme that makes sense for the controls on the form that you are customizing. Theme Manager does not check the theme to make sure that its customizations apply to the controls currently on the form.