The 'best' solution will depend largely on how your client will decide which pages are white-on-black and which are black-on-white.
Will it be alternating sub-pages in a parent page; parent pages in one, sub-pages the other; simply by ticking a box in the CMS for that page; or are they different page types (Page sub-classes)?
Personally, I would avoid creating additional templates and css files and go for setting the class of the main element (body or containing div) using a variable in the template Page.ss e.g.
<div class = "typography {$BlackOrWhite}">
Then you decide the value of $BlackOrWhite in your Page class depending on the conditions above and style for class black/white accordingly in your layout.css
Rich