Make show-paren-mode less subtle

It was often really hard to see matching parentheses when I wanted to.
The use of darkturquoise in the background is a variation on the
built-in default, which uses turquoise.

Fixes dbrock#4

zenburn.el

@@ -259,7 +259,7 @@
      '(highlight ((t (:underline t))))
      '(paren ((t (:inherit zenburn-lowlight-1))))
      '(show-paren-mismatch ((t (:inherit font-lock-warning))))
-     '(show-paren-match ((t (:inherit font-lock-keyword))))
+     `(show-paren-match ((t (:background "darkturquoise" :foreground ,zenburn-bg+2))))
      '(match ((t (:weight bold))))
 
      `(cursor ((t (:background ,zenburn-fg :foreground ,zenburn-bg))))