You can change the colour of the background by following the steps below:
1. Click on the 'Library' icon.
2. Click on the book you would like to read.
3. Select 'Read'.
4. Click on 'Aa' at the top of the page.
5. Choose the darker colour.
6. The background is now darker with the writing in white. You can switch back to the previous colour combo by clicking on the other option again.