Here is dark theme I knocked up, based loosely on Visual Studio's Dark theme which uses dark grey and blue shades instead of black which I find more eye friendly.
Edit: 2017-07-13 Included the fix for the column headers and toolbars.
So, I am enjoying this theme but I realise that the image preview pane for images are still in blinding white. Is this not something that can be skinned @Stumpii ?
When no image is loaded: Preferences / Display / Colors and Fonts / Viewer pane background (or a background image set in the Images page if you want something more complex than a solid color).
When an image with transparency is loaded, or one which doesn't fill the pane, the background behind it is set via Preferences / Viewer / Viewer Pane / Picture background color.
Yeah, wish I'm at home and can show you a screen shot. Basically the fourth pane to the right of the "Destination" window. This is the one where it will preview a doc or photo when the file is clicked, not the window that was launched when a photo is double clicked.
There seemed to be some inconsistencies. I am not sure if it was due to skinning or if it was a bug but the pane will turn white in certain instances (which I can't reproduce but I have captured the screenshot) and always in Recycle Bin and Network Drive.
You could try Preferences / Display / Options / Enable background images in virtual folders but it doesn't always work that well, especially with dark themes.
A few folders are delegated to the Windows shell (Explorer, essentially). Folders where Opus cannot really add any value as the only thing you can do in them is double-click something to go somewhere else. The Windows shell does not provide many options to change the way it looks, so those folders will look like the rest of your system, not like Opus if you have Opus configured to look non-standard.
Dumb question: if I want to revert to default white "theme", how do I do that? I go to Settings-Lister Theme and only see this one theme I have installed. There is no default to switch to. Will be useful to have one to switch to, actually.