Slideshow update (theme and captions)

Switched Nivo theme from “default” to “light” because the arrows on the
“default” theme were annoying (they show up when you hover over the nav
control buttons) and I started getting annoyed at how the whole thing
goes dark when you hover.

Also updated the captions so that they just appear at the bottom rather
than taking up the whole slide. I think it’s less intrusive. I’m hoping
that now I can use Nivo on mobile as well; I’ll change that on the next
commit and see how it looks.