Can you please post your prefs.ini file? (This file is located in /Documents/My Games/GalCiv3) This file will allow me to test with your exact settings, which may help reproduce the problem here. Thank you.
Can you please verify that you have the following file in your directory? /Galactic Civiliations III/Gfx/Shaders/Starbase4.fxo Also, please try the "Verify Integrity of Game Cache" option in Steam. Right-click Galactic Civilizations III in your library, and go to properties. Go to the "Local Files" tab and click on the "Verify Integrity of Game Cache" button.
I have a suggestion that may help. Please try deleting (or moving to a backup folder) the Atlas.dxatlas file in your /Galactic Civilizations III/Screens/Atlas folder. Then restart the game. If this still does not help, please send us your debug.err file. Thanks.