Create contrast & font size user controls

Site URL: https://fossilfueltreaty.org

Has anyone successfully added user accessibility controls for website visitors to toggle to a high contrast version of a website, or to increase font size? 

Hoping to add something similar to the sticky controls on left hand side of https://pwd.org.au/

Any tips or guidance would be appreciated! 

I think I saw some SS sites, have this. Can't remember what site it is.

You can post on some Facebook Groups

• Squarespace Rockstars
• Squarespace Entepreneur
• Squarespace Customization Resource Group