It’s probably by design.
Adapting the Theme to your wishes is one way to go, but you’ll probably be better of selecting a different Theme.
The Mellow Yellow Theme is meant to be the High Contrast Theme, but I find it to be not that esthetically pleasing…
Have you checked the Grey Matter Dark and the Solarized Dark Predefined Themes?
The Midnight Theme’s creator has bad eyesight and developed the Theme for himself.
Try other Themes from the Scrivener 3 Themes for Windows topic.
HTH