TV interface
sonic-tv-interface provides SonicDE's interface for televisions. To use it, choose SonicDE TV from the list of sessions on the login screen.
On-screen keyboard
When a text field gets focus, for example the search field, the on-screen keyboard opens. That works with a remote or game controller too, so you can type without a physical keyboard.
Third-party applications
These applications aren't part of SonicDE, but they suit a TV.
Kodi
An open-source media player and all-in-one entertainment hub for local media. See the Kodi website.
Plex HTPC
Plex is a streaming platform and media server that can be hosted on a personal server and reached from anywhere. It also has on-demand streaming without a personal server. Plex HTPC is a variant designed for a home media experience.
Steam
A popular gaming platform and store. Steam's Big Picture mode gives a TV-friendly interface for your games. See the Steam website.
Big Picture mode reads input directly from the controller, and links the controller's Guide button to the Steam menu. This can cause double actions when you use the controller. Consider suppressing controller input in Steam, and turning off the home overlay link to the Guide button for your controller, in Settings > Input.
VacuumTube
A web wrapper for YouTube's interface for TVs, with several improvements such as an ad blocker and phone casting support. See its source code.