Hi, I've been following this discussion, and I'll propose a little different approach. We're discussing here about switching themes for different kind of manipulations (dark for better colour perception, light for better on- paper look, etc.). I think that, as Sven pointed out (from elle's comments), that the padding color is the most important adjustment piece, not necessarily the theme. If your theme is neutral, middle-ish gray and you can tweak the padding color, then you don't have to switch themes for different kind of manipulations. In that case, having a tool for fast-switching the padding color would be, in my opinion, far more interesting and useful than a theme switcher. I've been working with graphics programs for years, and it never crossed my mind to switch themes "per project" and believe me, I do know how a theme can affect your perception. For instance, I always end up tweaking the curves of images that are inteded for print, as the dark UI of darktable makes them look brighter. But I wouldn't fix that by making darktable's UI lighter, as I would be producing washed out images for other targets. So, in my experience I found that middle gray in the padding area is a reasonable default, while being able to choose quickly between darker or lighter padding would be a really useful per-project setting. Once you have the appropriate padding color the UI theme doesn't matter much (as long as it's achromatic).
And regarding special themes for print, I think it's a flawed approach. In a pre-press environment you tweak your screen white point to match the white point and white level of white paper under controlled lighting. So you can't predict if your screen white is brighter or darker than paper white, so picking the white level for the UI elements is just guessing. And a white UI is visually exhausting too, so it's not really a good idea for a theme. Gez