So, for Extension:Themes I understand this approach, but for core, this is bad advice. Overloading another skin creates all sorts of problems with specificity and also with duplicate CSS that is unnecessary for many users. The right advice is that if you have a dark theme, you should have a light theme (and if you want to call that puppy default, then I don't mine), and the rest of the skin should only have CSS that is shared by both themes.
Topic on Talk:Requests for comment/Themes in core
The fact of the matter is that if you're using CSS, you can use it as badly or well as you like, regardless. You see this presently in the implementations of responsive skins: some have separate styles for different resolutions using breakpoints (ideal), but many simply do full styles for one resolution and then only use the breakpoints to override those for others, or don't even use breakpoints at all (instead depending on mobile site, js, whatever). The problem is, for themes, the latter sort of appraoch is currently the only option.
This RfC is basically about adding the option to do a better job of it, allowing separate styles for each theme without having to override everything after the fact as is pretty much required using only an extension, but it won't guarantee devs actually do that, any more than having support for media queries guarantees devs actually use those well to implement responsive styles.