You need to use a text editor to change a text file and you need to rename two files. If you want 60fps mode you need to download a patch. Instructions below.

Go to desktop mode, go to https://www.gamepressure.com/download.asp?ID=69368 and download the 60fps exe.

This download is called swtfufpsfix17022015.zip and you only need to extract the .exe file thats inside this zip in the 60fps subfolder.

In steam os desktop mode in Discover app (the blue shopping bag icon), in the search bar beside the home icon on the top left hand side type "text editor" and install a text editor.

Open dolphin (the folder explorer that steam os uses), click the burger style (3 horizontal lines) button on the top right hand side to access dolphin options and make sure Show hidden files is ticked.

Under "Devices" on the left hand side of dolphin file explorer (you may need to scroll down), select home, which will allow you (by clicking the "deck" folder) to navigate to /home/deck/.steam/steam/steamapps/compatdata/32430/pfx/drive_c/users/steamuser/AppData/Local/Aspyr/Star Wars The Force Unleashed and open config.xml with the text editor.

Set video width to 1280 and height to 800 by replacing the numbers listed in the respective tags. Change IsKBAndMouse to FALSE (just delete true and type FALSE). Now close the file and save.

Scroll down to removable devices in Dolphin on the left hand side and choose "primary" then navigate to steamapps, common, star wars the force unleashed. Put the 60fps exe here. There is a file in the folder called SWTFU Launcher.exe. rename it to SWTFU LauncherOLD.exe

Then rename the exe from the 60fps folder as SWTFU Launcher.exe

It should work in desktop mode and also gaming mode now.

If you don't have the 60fps exe download, just rename the file that already exists in the same folder called SWTFU.exe and call it SWTFU Launcher.exe

This will work with controller and will launch properly but it will be in 30fps.