The display contrast slider on my MacBook Air doesn't work

Best to try it yourself. That way, you'd know, which place in the slider is good for your eyes. Open some app, for example Finder, or some file so you can see the change. I have kept the display contrast slightly higher, as I have an eye problem. Here's my setting.

I have also changed the text size to match my vision.

That slider is actually increasing the contrast, not decreasing, that is, if you push it to the right.