What should a well-designed high contrast function do?

I want to make my website accessible for nearly everyone. I already have a function to increase font-size. Now I am doing the high contrast function.

However, how should such an interface for visually impaired look like? I already asked another person who is visually impaired and he said light text on dark background is better for him. And in regards to color: He likes dark green.

I would appreciate it if you could give me some other best practices to make the visit of my website a good experience for visually impaired.

Should I also change the font of my website? If yes, what type of font?