Please attempt to follow the instructions on https://www.mediawiki.org/wiki/Skin:Vector-DarkCSS. DarkVector is based on that and is better because it will support up-to-date vector as well likely. Therefore I am declining this. If this does not work for you with just that css then it may be considered but please try that first. Thanks!
The vector CSS you linked to is out of date, as mentioned in the Discussion page on MediaWiki. Another user explicitly recommends using the skin I linked to instead. Even if the custom CSS worked, the CSS would not apply to certain restricted pages, resulting in the user being blinded by a sudden light theme.