gerrit css tweaks to make it more like old gerrit

the front page had extra space per each gerrit change line. Set
the change height back to 1em instead of 20px to provide the more
compact view again.

the coloring of non voting blocks is visually distracting from the
actual votes, so reset their background to be whatever is behind
it.

Change-Id: Idcbe48b204af058e4530298c7098bdff69247a60