Where is the stylesheet code?

The most common would be /wp-content/themes/your-theme-name/style.css but this may not be the case depending upon the theme you are using.

The easiest way to find out is to use Chrome Developer Tools.

In Chrome open your website and right click on the element you would like to alter the CSS for, then click ‘Inspect Element’. The CSS files and classes which apply to that element will be listed in the right hand pane.