Change syntax highlighting theme to base16 Ocean Dark

Change the current syntax highlighting theme of docs.gitlab.com to base16 Ocean Dark

Closes https://gitlab.com/gitlab-com/gitlab-docs/issues/168

Review Apps:

• http://syntax-highlighting-monokai.178.62.207.141.xip.io/ee/ci/examples/devops_and_game_dev_with_gitlab_ci_cd/index.html
• http://syntax-highlighting-monokai.178.62.207.141.xip.io/ee/ci/examples/artifactory_and_gitlab/index.html
• http://syntax-highlighting-monokai.178.62.207.141.xip.io/ee/ci/examples/php.html
• http://syntax-highlighting-monokai.178.62.207.141.xip.io/ee/ci/yaml/README.html
• http://syntax-highlighting-monokai.178.62.207.141.xip.io/ee/ci/examples/sast.html
• http://syntax-highlighting-monokai.178.62.207.141.xip.io/ee/topics/git/numerous_undo_possibilities_in_git/index.html