minor LESS CSS fixes (zero units, dimensions)

A few zero values included units, and some margin and padding dimensions
had redundant values on the end.  Regarding the latter, since CSS 1 "if
there are two or three, the missing values are taken from the opposite
side".  Leaving values off hurts clarity (I can never remember if the
order is top-left-bottom-right) but both Chromium and Firebug's CSS
inspector will display
   margin: 0 0 15px 0;
as
   margin: 0 0 15px;
anyway.

Bug: 57083
Change-Id: Ic51b2ac196a434857f19d51cd2a92686d53c4877