1) Using the Additional CSS area in Customize
This is the preferred way if you only need to make a few small changes:
and
You can also check these articles:
https://elementor.com/help/how-to-use-selector-in-the-custom-css-tab/
https://elementor.com/help/global-custom-css/
https://elementor.com/academy/how-to-add-custom-css/
https://elementor.com/help/custom-css-pro/
2) Creating a child theme
This is recommended if you want to build your CSS on top of ours, but don't want to use the Custom CSS Area in the theme options.
You can find a sample child theme here: http://vamtam.com/child-themes/
The official WordPress documentation explains this topic really well: http://codex.wordpress.org/Child_Themes
Please note that you should always have a copy of your custom code so you are able to restore it anything goes wrong.