This sets the variable to the background value of
$fish_color_search_match, which fixes the case where you switch from a
theme with a set selected background (like our default, now) to one without.
These are simple
var val [val val]
files. Basically the bit in `set -g fish_color_escape 86c1b9` after
the `set -g `. Since we're not going to `source` them, however,
arbitrary code and expansions are unsupported.
Also comments and such don't currently work.
This allows them to be easily readable both from webconfig (next
commit) and the shell (later).
