Everything else in Kitset is designed so you never need this. It exists because there is always one thing: a particular spacing on one page, a hover effect a client insists on, a tweak for one screen size.
What you write applies to that site only and survives publishing. It is not checked or corrected, so it is the one part of Kitset where you can make your own site look wrong. If something goes strange after you add a rule, take the rule out first.
Step by step
In the editor, open the custom CSS panel.
Write your rules.
Watch the page as you type. It applies live.
Publish when you are happy.
If it does not go to plan
My rule is not doing anything.
Usually specificity. Our styles are reasonably specific, so a bare element selector often loses. Target the class you can see in the inspector.
I broke the layout.
Clear the panel. Nothing else in your site depends on what is in it.