Your iMac is supported (for Steam/GOG, late 2013, with AMD GPUs, is also supported).
How is the background in the main menu? Is that also very dark?
Double check that HDR is disabled: hold the Cmd button on game startup to get to the configuration menu, if it is not coming up automatically (which should be the case, with an option to turn off).
From a quick search it looks like the Radeon R9 M290X does have some HDR support, though, so as long you are also outputting to a display that has HDR support, this may be an issue with the settings. To calibration HDR, if you have not already done so: - Ensure you have Automatic Brightness Adjustments enabled (System Preferences/Display) - Go to Options->HDR settings - Start with Max Brightness and Min Contrast - Press ‘Apply’ - DON’T apply general video settings. Just click close button and Discard the settings; there is a known bug with this - next, adjust the HDR Brightness/Contrast settings to your preference
Try creating a new user account, switch to that account and try starting the game from there, from the app container (in /Applications).
Try doing a clean boot and test the game. Start/restart your Mac and hold the Shift key down as soon as the progress bar comes up in the startup window, releasing it after the desktop appears. Alternately, from the login window, hold the Shift key when you click the Log In button, and release it when you see the Dock.
