How to onstall a screen saver

Settings > Theme (may take a few seconds to load) > Screensavers will take you to screensavers for your Roku model from which to choose.  The ones from Roku and a few others are free, most are not.  Some are free but have purchasable add-ons like time and temperature displays.  Most give you the chance to view screen shots before you choose to install them.

You may find some additional screensavers via a search in the Channel Store. Some are from Roku, most are from other sources.