background colour for covered and uncovered squares in Mines, since
otherwise you have to distinguish them by the edge highlights alone.
So here one is; it's not _very_ different (it just looked odd if it
was any darker than this), but anyone who wants a bigger difference
can reconfigure it using the MINES_COLOUR_1 environment variable.
git-svn-id: svn://svn.tartarus.org/sgt/puzzles@5876
cda61777-01e9-0310-a592-
d414129be87e