Light text on a dark background. Easier in dim rooms and a little kinder on battery — but many eyes, especially with astigmatism, read it worse. Try both.
The drawing that proves the sentence above. It is the same card that is in the book.
Think of it as
reading by lamplight instead of floodlight.
For example
You switch to the dark screen at night and it feels kinder. Your friend tries it and says the words look smeared. Both of you are right.
This is one card out of 275.
The hundred words you meet most are free — the whole set, animated, in one file that works with no internet.
Volume 1 has all 275 in fifteen chapters, with Staying Safe the largest chapter on purpose.