Quote:
Those files have some text on the background, instead of a plain background.

The text is just to see what it would look like if it were in use.

Open up Dialogs->Layers and click the eye to hide the bottom text layer - then when you do a File-Save As... and select PNG, it will prompt you to export and save only the visible layers.

If you select the lens layer (I think it is named New Layer in that file) you can use the paint bucket tool to fill in different colors and tweak the layer transparency to make it look like you want.
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Mark Cushman