Sekia includes a child theme to make the updates easier. If you are planning to do several CSS changes, it is highly recommendable to do these changes inside a Child Theme, so you will never lose these changes. The steps are pretty simple:
Upload sekia.zip into wp-admin -> appearance.
After activated, you must upload sekia-child.zip into wp-admin -> appearance.
You can edit whatever CSS you want or PHP into functions.php.