Whenever you are using a custom theme or font, when Mac OS goes to Dark Mode, Accordance leaves the custom theme with light background. The other default themes like (Athens, etc) swap over no problem.
The opposite is true too. If you change your custom theme to have a black background, when you switch back to light mode it retains the dark background.
Maybe we need an option for a light custom theme and a dark custom theme that Accordance will automatically utilize if you have customs enabled.