Fix theme preview colors for the Zenburn theme

No associated issue, just a minor fix to make the Zenburn theme
use the correct colors for its button in the theme preview page,
rather than the default theme colors.

tildes/scss/themes/_zenburn.scss

@@ -43,6 +43,6 @@ body.theme-zenburn {
 
 @include theme-preview-block(
   "zenburn",
-  map-get($default-theme, "foreground-primary"),
-  map-get($default-theme, "background-primary")
+  map-get($theme-zenburn, "foreground-primary"),
+  map-get($theme-zenburn, "background-primary")
 );