At my age, I find most dark modes either hilariously garish / painful or so subdued I cannot tell where one title bar ends and the other begins - they're all the same color of "dark." All of the FT-mode software falls under that umbrella. The only "dark" mode I have found remotely useful is Breeze Twilight on my Mint box using KDE.
While not "infinitely configurable," you can come up with a "dark" mode in Fldigi:
?
Regards,
Joel - W4JBB
Joel Black - W4JBB