Switch to line borders for chess spaces.

The matte borders make me specify the thickness for each side. Since all
sides had the same thickness, I repeated the same number 4 times. I
should be using line borders instead, which only require that I specify
the thickness once. There is no other difference between matte and line
borders.
Add compound border for highlighting.

The straight line border looks weird when adjacent spaces all have
highlighting; they blur together. I think this looks better.