TemplateStyles is now installed, and we’ve started using it in a handful of templates. As long as you have permission to edit a particular template, you should be able to create a stylesheet for it.
By the way, these stylesheets aren’t only useful for adjusting to the dark color scheme. For example, you can also customize the color of links within the template’s output, eliminate an ink-wasting background on a printed copy of the page, or make a WikiProject portal rearrange its sections to fit a narrower screen.