Fix for cobalt URLs

Cobalt's css file was moved to presentations/dependencies/themes/cobalt/css/ from presentations/dependencies/themes/cobalt/.

This broke a few URLs, such as well, the CSS file, but more importantly, the awesome blacktocat watermark. Can't have a presentation without an Octocat!
Merge pull request #125 from github/cobalt-fixes

Fix for Cobalt presentation theme URLs