Attempt to improve focus ring styles in modern browsers.

Attempt to improve focus ring styles in modern browsers.

It seeems new browsers use a rather different focus ring style by
default from how it used to be.  This means the focus rings in the
clicky table headers (which are made by CSS rules) don't look anything
like the focus rings on links.

To try and fix this, let's explicitly style all the focus rings to
be more like how they look by default in new browsers.