A new citation style can also be added directly in the Zotero app:
Click on Edit in the top left-hand corner of the Zotero app.
Click on Preferences at the bottom of the drop-down menu that appears.
Follow the steps below, starting with the Get additional styles... step.
Click on the Zotero thread.
Click on Document Preferences.
Click on Manage Styles... in the Zotero - Document Preferences pop-up window that appears.
Click on Get additional styles... in the Zotero Preferences pop-up window that appears.
Type your citation style in to the Style Search box in the Zotero Style Repository pop-up window that appears.
The default Title Search phrase will automatically disappear and doesn't need to be deleted.
As you type in your citation style, styles will automatically be found.
You can hover your mouse over a citation style to see examples of Citations and a Bibliography.
Click on the citation style that you want to add.
Click on your newly-added citation style in the Zotero Preferences pop-up window.
Click on the OK button in the bottom right-hand corner of the Zotero Preferences pop-up window.