I believe the developers have misunderstood 'Dark Mode' in iOS 13.
Dark Mode is a system wide option in iOS 13 that means that the User Interface in apps should be in dark colours, rather than light colours. The Accordance app correctly switches the user interface on iOS 13 to a dark option when this is set system-wide.
However, Dark Mode should not affect the presentation of Content. It should be up to the user to decide what colour the content is on an app by app basis.
Unfortunately, Accordance for iOS removes the ability of the user to set custom colours for content – despite the fact that this used to work fine. If Dark Mode is on, it won't allow you to change the content out of 'night mode'.
As an example of an App which correctly distinguishes between User Interface and Content, please note Apple's own iBooks app – in the attached screenshot, the interface is in 'dark mode', but the content of the book is user customisable (in this instance, light colours).
I suggest Accordance for iOS be updated so that it respects the system setting for Dark Mode with regard User Interface, but allows the user to customise colours for content regardless of whether Dark mode is on or off.