ENH: Improve "list element" styling

Tweak the styling of extension "list items" slightly to improve
appearance (adding a hover effect and a subtle glow on hover). Adjust
extension screen shot "list items" to match the extension list, and to
give the screen shots a slightly less flat look.
Merge pull request #54 from mwoehlke-kitware/more-style-tweaking

Improve "list element" styling