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.