Clean up fourth.md from @fredmanglis

Make headers less severe (H1 -> H3) and clean up the post to make
better use of markdown syntax so we can see what the theme is capa-
ble of (and to see how other themes look with code blocks, etc).
Merge branch 'themes' of https://github.com/alanorth/nairobilug.or.ke

* 'themes' of https://github.com/alanorth/nairobilug.or.ke:
  Clean up fourth.md from @fredmanglis