Remove css rule that stops syntax highlighting (#484)

* Remove suspect css rule

Removed the ".post-full-content pre code * { color: inherit }" rule which prevents highlight.js from working.

* Changed to just exclude span

Less disruptive to just remove span from color inheritance.

* Remove extra newline

assets/css/screen.css

@@ -939,7 +939,7 @@ Usage (In Ghost editor):
     background: transparent;
 }
 
-.post-full-content pre code * {
+.post-full-content pre code :not(span) {
     color: inherit;
 }