How can I modify OSX GUI colors to be more readable for weakened vision
Neither MacOSX nor iOS offer to change the OS theme. If you want to do this, you need to use third party software.

What MacOSX does offer, is changing the color profile and calibrate it in System Preferences -> Displays -> Color -> Calibrate.

I've played around with different options and found the option "Cool bluish white" to be much easier on the eye: