I am guessing this is by design, since as I understand it, when properly booting games through Steam on the Steam Deck, the actual internal controller cannot be found by SDL, since Steam Input always hooks and hides it. From what I can tell, this cannot be disabled either (and that's probably for the best on Steam's part.) I am not far enough along in my own development to be acquainted with Steamworks, but I'm guessing that there are ways to determine what physical buttons are mapped to the virtual xbox360 controller, which could then be used to display the proper glyphs.

With that said; closing Steam in "Desktop mode" disables any Steam Input shenanigans, and running an application directly allows SDL to find and use the Steam Deck's internal controller. However, there's no SDL_GamepadType for this, so it just shows up as a standard controller, meaning this library could only detect and load Steam Deck glyphs via GUID. Currently, there is no guids file in the steamdeck folder, so this results in nothing being loaded.

I'm not sure if there are any hardware revisions that would change this, but I did grab the GUID of my Steam Deck's internal controller, which is 0300efefde2800000512000011017200. This could at least be added to the themes to solve this particular case, although in the grand scheme of "instant Steam Deck support" it leaves much to be desired.