@paperdigits I like dark theme too. It’s far more pleasant than neutral grey. However it’s not the point 
Here are my changes:
diff darktable.css darktable.css.old
1,2c1,2
< @define-color bg_color #777777;
< @define-color plugin_bg_color #777777;
—
> @define-color bg_color #212121;
> @define-color plugin_bg_color #252525;
6c6
< @define-color selected_bg_color #666666;
—
> @define-color selected_bg_color #353535;
11c11
< @define-color darkroom_bg_color #777777;
—
> @define-color darkroom_bg_color #333333;
14,15c14
< /**@define-color lighttable_preview_bg_color shade(@lighttable_bg_color, .5);*/
< @define-color lighttable_preview_bg_color @lighttable_bg_color;
—
> @define-color lighttable_preview_bg_color shade(@lighttable_bg_color, .5);
I find that the main difference is when previewing the image in fullscreen mode when no other colorful distractions are at view (like window frame, desktop theme and so on).
I prepare images for print so I’m more cautious than where preparing them for web.
It would to be able change theme in darktable