The most important is text size. Bigger text is easier to read. Make sure the user can resize the text. eg. no text inside bitmap images. You will probably hate

The most important is text size. Bigger text is easier to read. Make sure the user can resize the text. eg. no text inside bitmap images. You will probably hate me, but leave the text color and background to the default eg. don't set it. Let the user set it. Ok, so now you basically got black text on white background. But you can still play with spacing! And you can play with text size. So instead of using contrast, use text size and spacing!