accessibility should not be a default off preference.
Couldn't agree more. There's no need to create a user setting when we could just enable a11y features for everyone. I can't think of examples where making something accessible for, say people with colour-blindness would negatively impact other users' experience.
Quick mockup with exclamation mark for reference: http://codepen.io/awesomephant/full/LJkBm/
Best, max @awesomephant