css/local.css: Add local CSS overrides

css/local.css: Add local CSS overrides

For styling that's more than a one-off for a given element.  Here I'm
overriding the header alignment, which was previously defined in
css/reveal.css:

  .reveal table th,
  .reveal table td {
    text-align: left;
    padding-right: .3em;
  }

Left-aligne headers over images (on the us-vs-them slide) looks funny
to me.  We could align the images (in <td>) too, but I think the
current table looks fine with them left-aligned, because the Garnet
image is full-width, and the Medusa image is almost full-width.