Ereader-friendly colour scheme

[AmrSaber]

Al salam alaykom,

This issue is related to my comment on #648 but I think it should have its own issue.

On Android ereaders, displaying a colored background (that gets transformed into grey-scale) is a slow and heavy process.

Would be great to have an "ereader mode" option -or something similar- that changes the background color to white and text colour to plain black (so basically the opposite of dark mode) to provide a better experience on ereaders.