Shadows, and Shadows-based ReadyThemes, ship with the A11y Toggle extension included. This is an accessible replacement to the checkbox-hack for toggling sections. The CSS is part of /themes/%STORE_ID%/THEME_NAME/extensions.css and the JavaScript is in /themes/%STORE_ID%/THEME_NAME/extensions.js. If you want to make styling changes, it is recommended you add them to your /themes/%STORE_ID%/THEME_NAME/theme.css file.
If you are using Shadows DE, you can modify the CSS or JavaScript in build/extensions/show-related/ and update as part of your build process.
You can find some additional documentation for the extension can be found at http://edenspiekermann.github.io/a11y-toggle/.