Go to your wiki's control panel. Look at the Keyboard Shortcuts tab. Toward the bottom is a "toggle-sidebar" record.
On the left side of the record line is a right-arrow. Click the arrow to expand the keyboard shortcut sub-records for each ooerating system type. Use the pencil icon to assign a different keyboard shortcut.
You can also create your own keyboard shortcut or button to call an action to show or hide the sidebar. You can use the command <$action-setfield $tiddler="$:/state/sidebar" text="no"/> to hide the sidebar, or "yes" to show the sidebar.