🐛 Fixed incorrect/missing colours for various elements

closes https://github.com/TryGhost/Casper/issues/548
- the recent postcss upgrade resulted in real CSS variables being output with a fallback mechanism, unfortunately this broke a number of colors throughout the theme
- disabled the fallback mechanism to revert back to the previous behaviour

gulpfile.js

@@ -32,7 +32,7 @@ const handleError = (done) => {
 function css(done) {
     var processors = [
         easyimport,
-        customProperties,
+        customProperties({preserve: false}),
         colorFunction(),
         autoprefixer({browsers: ['last 2 versions']}),
         cssnano()