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

There are several 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
If you wish to add custom CSS only to specific pages of your website, you can use the custom CSS feature of Nimble Builder, a free page builder for WordPress.
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 attributes to modify? Use the developer tools : press F12 in your browser and inspect the page code.