Issue: Book "sidebar" sections keep background color when switching between light/dark themes

Platform: Linux

Settings:

Default Dark theme and Default Light theme

Steps:

1. Find a book with a sidebar section (this one has a grey background).
2. Switch between themes that change the font color (i.e., Default Dark/Default Light)

Suggested Correction

When switching modes, change the background of this element. (Perhaps inverse(1) or allowing for custom color setting when designing themes).

Screenshot with a more "expected" representation.