1) Using the Custom CSS area in Customize > Additional CSS
This is the preferred way if you only need to make a few small changes:
If you would like to add a specific class or ID for any element, you can do this by first wrapping the element in a Column, and then adding your chosen class/ID to the respective column options.
In the case of the ID, you need to enter an alphanumeric string without any spaces. In the case of the Class option, you need to enter a space-separated list of classes without dots (similar to how you would use HTML's class attribute). If you have entered my-column-class as the column's class, you can then use the following CSS selector for this column:
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 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 codes as sometimes you may mistakenly reset theme options to default and lose all your custom css.