Generating all those .desktop files is only half the battle. Now we need to tell EmulationStation how to use those suckers. We'll need to open the configuration file es_systems.cfg to add a new system for our Steam games. It might live at /etc/emulationstation/es_systems.cfg or at /opt/retropie/configs/all/emulationstation/es_systems.cfg. Once you've got it open, add this stanza to the file.
EmulationStation Windows
This tells emulationstation that there's a new system called steam which it should present to humans as Steam whose games live in /home/yourusername/RetroPie/roms/steam. Be sure to change that to whatever directory you put your .desktop files in earlier. Those game files will have names that end in .desktop, and when EmulationStation wants to run one it will run xdg-open on the selected game file. xdg-open will in turn open up that .desktop file, read the line Exec=steam steam://rungameid/$app_id, and ask steam to run the game with the target app ID. EmulationStation -> XDG -> Steam. The platform line tells any metadata scrapers that when they search for a game in this folder they should search for PC games. The theme line tells EmulationStation that if a theme supports Steam it should use that part of the theme for this system's collection.
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