Steam has just detailed it's client beta update, and among the extensive patch notes is the announcement that you can now customise x-input controllers.
The post says: "Added Xbox 360, Xbox One, and Generic X-Input controller configurator support. This allows all recognised controller types to use the advanced mapping features of the Steam Controller Configurator. Note that because X-Input currently lacks per-controller means of unique identification, all controllers of that type will share personalisation and configuration settings. As they share the same inputs, Xbox 360/One/Generic controllers will all see each-others configurations when browsing. Automatic conversion will be attempted when loading configurations from other controller types."
For full patch notes, visit the official page, as many more things have been changed, including adding support for using overlay keyboards for games that have launchers. Do you want to customise your controller?