Some of the standard themes in Gomo, such as Turing Blue, have been updated to have a screen background image by default. You can remove this using the following steps:
Open your project and go to Settings/Theme. Click on Customize theme settings. Now find the Screen image section in the Backgrounds section and click the browse button:
Now select the None box:
Click the Apply Changes button at the bottom of the screen.
That should remove the default background image from all your screens.