PostCSS parses CSS, runs plugins and outputs CSS. Yep, of course there is a plugin that does variables! And the "Myth" tool, mentioned in a comment here, is just a pack of PostCSS plugins.
And the OP uses PostCSS. Apparently, a lot of people here didn't read that part...
And the OP uses PostCSS. Apparently, a lot of people here didn't read that part...