Tested on build 1.9.0(12)  on iOS 13.7

The readable green in navigation headers renders lime green (accessibility?) upon return to white background if user has previously set background to black.

Steps to reproduce:

1. read book
   2. tap font-change icon
   3. change background to black 
   4. while background is black, use the TOC nav to move to a different page in book.
   5. switch background to white, and nav header will remain lime (not the original "OE green")

note: the lime header follows user to book detail page once it's been triggered.
note: leaving the book and hitting read again resets nav header to correct green.