In general the kind of thing you are talking about on the Mac is app specific if the app developer thinks it should be an option. From way back Apple's design philoshy has been "don't provide the user with too many pointless choices" and Microsoft's has been "allow the user or, more likely, the sysadmin to choose".
By the same token there's no way to do this for the web. (you can override style sheets willy nilly but don't expect a lot of stuff to work).
I suggest you might need to get used to reading text on backgrounds that aren't a "pleasing greenish" color, write all your own software, or learn to live in a world of pain.
Instead of allowing people to set the default edit field background to magenta (which windows does) Apple spent engineering effort on, say, universal access.
