Twenty Eleven: ensure `twentyeleven_url_grabber()` is pluggable by wrapping in a `function_exists` check.

git-svn-id: http://core.svn.wordpress.org/trunk@23474 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Ten: better color contrast for links and text, props elfin. Fixes #14794.

git-svn-id: http://core.svn.wordpress.org/trunk@23475 1a063a9b-81f0-0310-95a4-ce76da25c4cd