You can change the colour of the background by following the steps below:
1. From the "Home" page, navigate to your Library.
2. Tap on the title you would like to read.
3. Select 'Read Blinks'.
4. Tap on 'Aa' at the top of the page.
5. Choose the darker colour.
Additionally, you can enable the dark mode for the entire app by the following way:
1. Go to your phone's settings.
2. Scroll down and choose 'Display & Brightness'.
3. Select the 'Dark' option.