Add logger-theme.{c,h} with the logger plugin contribution to the
built-in "light" theme: 2 entries (logger.color.backlog_end=darkgray,
logger.color.backlog_line=darkgray) tuned for a light-background
terminal.
Same 2D-array pattern as the other plugin contributions.
Default option values are NOT changed.
Sébastien Helleu