Save and restore 'background'

This is necessary to support colorschemes with both light and dark versions, such as solarized.

I tested this with the solarized scheme.  I can reproduce #6 at `HEAD`, and this PR fixes it.
Maybe not a huge deal, but rather than assume maktaba#value#SaveAll will return a dict I'd prefer to treat the return value as an opaque type with no assumptions about it except that it will be accepted by Restore(). Passing an empty list to SaveAll does seem a little silly, but no more than explicitly sending an empty dict to Restore(), etc.