The Xoops templates can be installed fairly easy from the Xoops Administrator Area->System->Templates:

First you should find a suitable template in the Internet and download it on your local computer. Then you should use the “Upload a template set” functionality in Xoops. Click on the Browse button and navigate to the chosen template. Then click the Upload button.

After this you should click on the Generate buttons besides your Modules in order to create links for the new template:

Once you are ready, navigate to the Xoops Administrator Area->System->Preferences->General Settings and pick the new default template:

Then click on the Go! button and the new template will be set as default.