Ah, I see. You can create Themes on Mac in the same way - the theme files are not platform-specific.
Instead of "C:\Users\Appdata\Local\PokerTracker4", the base path for custom themes would be "/Users/<USER>/Library/Application Support/PokerTracker 4".
You can get to that folder via File > Open User Data Folder in the PT4 menu.
Note that the "Themes" folder does not exist there by default (on either Windows or Mac) as initially there aren't any custom themes.
The default theme files can be found inside the PokerTracker 4 application package contents on Mac, if you want to see some examples. Do not change the files here though.
Right click the application and choose Show Package Contents, and navigate to Contents/Resources/Data/Themes.
(On Windows the default themes are at C:\Program Files (x86)\PokerTracker 4\Data\Themes.)
If there is anything else Windows-specific in the guide that you're not sure about please ask.