Actually, if you are going for a dark theme, it might be better to use FANSUN. You can enable it on the dev wiki by going to the Gadgets tab in your preferences. There is a section called Theming where you can check the box next to the dark theme.
I see. If you are just trying to make it black, then you don't need to use an image at all.
Would that work without setting the background-image to none? background-image is supposed to go above the background-color. I have also found that sometimes imortant! is needed to override the background-image.
In the particular case Vozhan is talking about, yes; because the dev wiki has no background image. However, on a wiki that does have a background image, you are correct. You would have to set background-image to none.