How to change the style ( CSS code ) of the Customizr WordPress theme?

There a two recommended ways to customize the style (  CSS code ) of the  Customizr WordPress theme :
  • For small chunks of code, you can use the live Custom CSS section in the customizer panel
  • For experienced developers willing to make important customizations, it is recommended to edit the style.css file of a child theme

How to identify the CSS atributes to modify? Use the developer tools : press F12 in your browser and inspect the page code ( see this tutorial for firebug)