After install, go to Dashboard -> Pages & Themes -> Multi Themes and configure the add-on choosing the parent page and the theme you want to use on that page and on all children.

Do this to any pages you want and you can have different themes to different areas of your site. You can change the default site-wide theme, and Multi Theme Settings will not be affected.

If you want to remove some configuration, click on the trash icon on the right of the list on the bottom of the page.