Microsoft has just released a new update for the Xbox App on Windows 10 with some long requested features, including support for a light theme. This is probably done ahead of the next big update release for the Xbox One console, which will add a similar light theme option for the entire UI.
Microsoft includes a number of pre-installed Universal (Metro) apps in Windows 10. One such app is Xbox. It allows you to track your Xbox social activity and achievements, record game clips, and stream games from Xbox One to PC. The app connects to the Xbox Live community where you can chat with friends, share captured screenshots and gaming videos and play multiplayer games common to the PC and Xbox One. If these things are not for you and you have no use for the Xbox app, here is how you can delete it completely.