The Subsideincludes 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 subside.zip into wp-admin -> appearance.
• Activate Varn.
• After activated, you must upload subside-child.zip into wp-admin-> appearance.
• You can edit whatever CSS you want or PHP into functions.php.