Sydney Theme sidebar compatibility
As of March 2017, the recommend method for sidebar compatibility is to use LifterLMS Labs: Super Sidebars, a free add-on that allows you to add sidebar compatibility to just about any theme in a matter of seconds without having to write any code. See the full tutorial here.
Use the snippet below to add sidebar support to the WordPress Sydney Theme.
This code can be added to your themes functions.php file or can be added using a custom plugin.