How to use Sydney child theme, its important to activate a child theme for any modification.
Thank you for contacting us. I am happy to help with your queries.
Here is the general information from WordPress.org about child theme and how to use it. Any child theme works the same way, including Sydney child theme.
Further adding, we have a very basic child theme for Sydney, that can be downloaded from this link. And you can use it as a starting point.
I hope this reply helps.
Please let us know in a new topic if you have any further questions, or if we can provide you with any other assistance.