There are two options for setting the background color of Viewer Pane:
- Preferences -> Colors and Fonts -> Other Colors -> Viewer pane background
- Preferences -> Viewer -> Viewer Pane -> Picture background color
The two options do almost the same function except that the first one applies to the background of the Viewer Pane when no file is selected and the second one applies when some file is selected.
The second option is not changed when a theme is applied. And that's a problem. The second option could be eliminated so that the first option does both functions as it is modified when a theme is applied.
It would also be possible to apply an image to the background of the Viewer Pane when an image was being viewed.