I can add buttons to the blacklist that are created with FancyMenu, but buttons of the mod itself (including their start screen mod) are darkened by mistake, as well.
No idea if it's fixable but I thought I report it. FancyMenu is planning a large update soon, so things might be different after, but they might also be still not working then, too.

1.18.2 Forge 40.2.14
1.1.3 DME
2.14.9 FancyMenu

bugged

versus non dark mode

I tried adding any listed parameter to the blacklist, but to no avail.