I am using Windows 10 preview Build 18956.
I have my viewer pane set to display a black background.
When I view the contents of a networked drive the display background is white.
This issue existed within an earlier version of Windows when using their File Explorer but was fixed by Microsoft. I am thus not sure if my current problem is related to File Explorer or Dopus.
I'm assuming you mean the file display, not the viewer pane. (The file display displays a list of files and folders within the current location. The viewer pane displays a single picture or document.)
Folders like Network are delegated to the Windows shell, since we cannot add any value to them (other than pure cosmetics, which might be a reason to handle them ourselves in the future, but is virtually the only reason to do so); all you can do in them is double-click a folder to go somewhere else.
The Windows shell does not provide any way -- documented, at least -- to tell it to use "dark mode" or custom colors. We can replace the background color by painting it ourselves, and there is an option for this, but it's not useful for dark themes because we can't replace the text color, and you'd end up with black text on a black background.