There are two options for setting the background color of Viewer Pane

Help & Support
1

There are two options for setting the background color of Viewer Pane:

  1. Preferences -> Colors and Fonts -> Other Colors -> Viewer pane background
  2. 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.

2

That's by design.

One color is the background behind images that have alpha/transparency/translucency.

The other background color (which can also be combined with a background image) is for when nothing is displayed in the viewer.

You mean an Opus theme? We can fix that if so. There are a few colors which themes don't set at the moment which we need to fix, after it was reported quite recently.

3

Yes, I do. The option "Picture background color" has no possibility to choose transparency:

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Imagem%20da%20%C3%A1rea%20de%20transfer%C3%AAncia759×687 36 KB

And when you apply an Opus theme, the background color behind images does not change. It's still white when theme is dark.

I think it's better and easier to add the possibility of choosing transparent color in "Picture background color" option than to add this option to theme files. Thus, background color or background image of viewer pane would be visible.
If you can add transparent color, I would really appreciate it.