It seems like this cannot be said enough: accessibility overlays are not accessible. They're not good for people that need accessible websites. They're good for companies that want sites that look accessible for little work, and for the snake oil salesmen that sell them.
You may think "snake oil salesmen" is harsh, and these companies genuinely care about making the web more accessible. I stopped believing that after a major voice for web accessibility pointed out serious flaws in what they were doing, and they sued him to shut him up.
So I'll say this for anyone else on the Internet that hasn't heard it yet: accessibility overlays are bad. Please do not use them.
Read Accessibility overlays are not for disabled people by Martin Underhill.