Try "Reading Mode"
While you're mostly enjoying stock Android 9 Pie (or Android 10 Q) on this phone, OnePlus does add some helpful software tricks to the experience. One of those is the Reading mode, which comes in handy whenever you want to read. This will improve battery life, too.
Reading Mode turns the entire screen to grayscale, and gets rid of any colors. It basically mimics an e-reader, which makes reading books, magazines, or websites easier. We mean easier to read, and easier on your eyes, especially at night. It's a lot more than just a "read-mode" like we've seen on other phones and recommend owners use it.
Go to Settings > Display > Reading Mode and toggle on the Reading Mode Slider. Then, you can customize it so certain apps like the Kindle app instantly start in Reading mode.