#1654 Implement "dark mode"

[Ahasuerus]

Implement "dark mode". This functionality was requested on the ISFDB off-site blog -- see https://isfdb.blogspot.com/2025/08/2025-08-26-unscheduled-downtime.html , which says "Could You please work on a Dark Them layout for those of use who have sight issues. I personally can't stand bright backgrounds anymore. I would like if it had black background with white letters. "