The Old Reader - Has a Firefox Dark mode extension https://addons.mozilla.org/en-US/firefox/addon/the-old-reade...

Scrivener - Can be styled to have a dark mode and the newest releases for iOS have one you can select.

Sabnzbd - Has a night mode for their web interface

Boostnote - Has a variety of themes. Great for markdown notes and coding notes.

SumatraPDF - Allows customization of the color scheme in the Advanced options section. Requires knowledge of HTML colors