I love Agenda and I’ve a simple suggestion stolen from the official Mail app in MacOS.
I’m using the dark mode in Mojave but I think it can be really confusing in some situations when you have an application with a specific space with a note and a rich editor.
In the dark mode inside Agenda, everything is completely dark. I think it could be an option to have everything dark but maintaining the single boxes of the notes in white.
The Mail app in MacOS does the same thing. If you select the dark mode, everything is completely dark by default but there’s an option in the Preferences (“Viewing” tab) to not use the “dark backgrounds for messages”.
I think it could be a great option to have with a very little effort.