This patch removes some CSS rules that make the datagrid header show a

white background in Firefox. Firefox interprets visibility: none with

a background colour set to not show the background colour. Removing

these rules does not affect otherwise affect rendering.

I viewed the Review Board dashboard, all review requests page, and

users page review requests and reviews grids on the following browsers:

Chrome desktop
Chrome desktop (mobile mode)
Chrome for Android
Firefox desktop
Firefox desktop (mobile mode)
Firefox for Android
Safari desktop
Safari desktop (mobile mode)