Add background to html for the accessibility plugin (#249)

Added a background colour to html. This enables the accessibility plugin to invert the backend colour.

templates/cassiopeia/scss/blocks/_global.scss

@@ -9,6 +9,10 @@
   }
 }
 
+html {
+  background-color: $white;
+}
+
 body {
   position: relative;
 }