ScreenRuler Mac is a visual software tool to assist with reading on the Mac. The ScreenRuler software provides a strip or ruler across the screen, which can have the contrast changed and have the background greyed, coloured or dimmed out. You can also simply tint the entire screen. ScreenRuler includes ClaroView. ScreenRuler also supports dual monitors.
More information: https://www.clarosoftware.com/portfolio/screenruler-for-mac/