The Quantify extension, included in Shadows, allows for the use of buttons to increase/decrease item quantities on the product and basket pages. We have added data- attributes to the input fields to accommodate the use of the Quantity Limits module available in the Miva Apps store. 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/quantify/ and update as part of your build process.